[chore] Makefile: add gencodeowners (open-telemetry#40073)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
This PR adds a target to the Makefile to generate the CODEOWNERS file
locally without a GitHub PAT.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#40072

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added myself to a random receiver as a code owner and ran `make
gencodeowners`. It generated the `CODEOWNERS` file correctly without
requiring me to have a GitHub PAT (I do not have one set up).

<!--Describe the documentation added.-->
#### Documentation
Added reference in `CONTRIBUTING.md` and in the `check-codeowners`
GitHub Action error message.

<!--Please delete paragraphs that you did not use before submitting.-->

Author: braydonk

.github/workflows/check-codeowners.yaml

@@ -59,4 +59,4 @@ jobs:
         run: |
           cd pr
           GITHUB_TOKEN=${{ secrets.READ_ORG_AND_USER_TOKEN }} ../.tools/githubgen codeowners
-          git diff -s --exit-code || (echo 'Generated code is out of date, please apply this diff and commit the changes in this PR.' && git diff && exit 1)
+          git diff -s --exit-code || (echo 'Generated code is out of date, please run "make gencodeowners" or apply this diff and commit the changes in this PR.' && git diff && exit 1)

CONTRIBUTING.md

@@ -193,7 +193,8 @@ status:
 // Package fooreceiver bars.
 package fooreceiver // import "github.com/open-telemetry/opentelemetry-collector-contrib/receiver/fooreceiver"
 ```
-- Type `make update-codeowners`. This will trigger the regeneration of the `.github/CODEOWNERS` file and the [metadata generator](https://github.com/open-telemetry/opentelemetry-collector/blob/main/cmd/mdatagen/README.md#using-the-metadata-generator) to generate the associated code/documentation.
+- Type `make generate`. This will trigger the [metadata generator](https://github.com/open-telemetry/opentelemetry-collector/blob/main/cmd/mdatagen/README.md#using-the-metadata-generator) to generate the associated code/documentation.
+- Type `make gencodeowners`. This will trigger the regeneration of the `.github/CODEOWNERS` file. 
 
 When submitting a component to the community, consider breaking it down into separate PRs as follows:
 

Makefile

@@ -387,6 +387,10 @@ gencodecov: $(CODECOVGEN)
 update-codeowners: generate gengithub
 	$(MAKE) genlabels
 
+.PHONY: gencodeowners
+gencodeowners: install-tools
+	$(GITHUBGEN) -skipgithub
+
 FILENAME?=$(shell git branch --show-current)
 .PHONY: chlog-new
 chlog-new: $(CHLOGGEN)