.Net Agents - Remove (Legacy) Experimental Agents Project and Samples (#7979)

### Motivation and Context
<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->

New _Agent Framework_ is here:

[./dotnet/src/Agents](https://github.com/microsoft/semantic-kernel/tree/main/dotnet/src/Agents)

### Description
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

Removing all legacy agent code and samples. This effort has been
superceded by the current _SK Agent Framework_.

Included "README" in place of the old project that points to the new
framework.

### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: crickman

dotnet/SK-dotnet.sln

@@ -199,10 +199,6 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "AI", "AI", "{1B4CBDE0-10C2-
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Memory", "Memory", "{24503383-A8C4-4255-9998-28D70FE8E99A}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Experimental.Agents", "src\Experimental\Agents\Experimental.Agents.csproj", "{5438D1E3-E03D-444B-BBBA-478F93161AA8}"
-EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Experimental.Agents.UnitTests", "src\Experimental\Agents.UnitTests\Experimental.Agents.UnitTests.csproj", "{4AD80279-9AC1-476F-8103-E6CD5E4FD525}"
-EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Experimental.Orchestration.Flow", "src\Experimental\Orchestration.Flow\Experimental.Orchestration.Flow.csproj", "{B0CE8C69-EC56-4825-94AB-01CA7E8BA55B}"
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Experimental.Orchestration.Flow.IntegrationTests", "src\Experimental\Orchestration.Flow.IntegrationTests\Experimental.Orchestration.Flow.IntegrationTests.csproj", "{3A4B8F90-3E74-43E0-800C-84F8AA9B5BF3}"
@@ -567,18 +563,6 @@ Global
 		{3FC4A81B-8ABE-473F-BC7C-6F4885775534}.Publish|Any CPU.Build.0 = Debug|Any CPU
 		{3FC4A81B-8ABE-473F-BC7C-6F4885775534}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{3FC4A81B-8ABE-473F-BC7C-6F4885775534}.Release|Any CPU.Build.0 = Release|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Publish|Any CPU.ActiveCfg = Publish|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Publish|Any CPU.Build.0 = Publish|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8}.Release|Any CPU.Build.0 = Release|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Publish|Any CPU.ActiveCfg = Debug|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Publish|Any CPU.Build.0 = Debug|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525}.Release|Any CPU.Build.0 = Release|Any CPU
 		{B0CE8C69-EC56-4825-94AB-01CA7E8BA55B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{B0CE8C69-EC56-4825-94AB-01CA7E8BA55B}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{B0CE8C69-EC56-4825-94AB-01CA7E8BA55B}.Publish|Any CPU.ActiveCfg = Publish|Any CPU
@@ -906,8 +890,6 @@ Global
 		{3FC4A81B-8ABE-473F-BC7C-6F4885775534} = {A21FAC7C-0C09-4EAD-843B-926ACEF73C80}
 		{1B4CBDE0-10C2-4E7D-9CD0-FE7586C96ED1} = {0247C2C9-86C3-45BA-8873-28B0948EDC0C}
 		{24503383-A8C4-4255-9998-28D70FE8E99A} = {0247C2C9-86C3-45BA-8873-28B0948EDC0C}
-		{5438D1E3-E03D-444B-BBBA-478F93161AA8} = {A2357CF8-3BB9-45A1-93F1-B366C9B63658}
-		{4AD80279-9AC1-476F-8103-E6CD5E4FD525} = {A2357CF8-3BB9-45A1-93F1-B366C9B63658}
 		{B0CE8C69-EC56-4825-94AB-01CA7E8BA55B} = {A2357CF8-3BB9-45A1-93F1-B366C9B63658}
 		{3A4B8F90-3E74-43E0-800C-84F8AA9B5BF3} = {A2357CF8-3BB9-45A1-93F1-B366C9B63658}
 		{731CC542-8BE9-42D4-967D-99206EC2B310} = {A2357CF8-3BB9-45A1-93F1-B366C9B63658}