test: Remove `net481` build targets, `[NetFrameworkTest]` / `[NetCoreTest]` attributes from integration test projects #5807

Job	Run time
Check if source files were modified, skip remaining jobs if not / Check if source files were modified, skip remaining jobs if not	17s
Validate shell scripts	16s
Build FullAgent and MSIInstaller	7m 43s
Build IntegrationTests	31s
Build UnboundedIntegrationTests	35s
Create Debian package	51s
Create RPM Package	51s
Run Linux Container Tests / Run arm64 Linux Container Integration Tests	44s
Run Linux Container Tests / Run amd64 Linux Container Integration Tests	51s
Run Unbounded Tests	1s
Run IntegrationTests	1s
Check Test Matrix Status	9s
Run ArtifactBuilder	1s
	12m 51s

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