Configure web_based_commit_signing_enabled with group update endpoint

What does this MR do and why?

Configure web_based_commit_signing_enabled with group update endpoint

References

• Add `web_based_commit_signing_enabled` to group... (#540922 - closed)

How to set up and validate locally

1. Simulate Saas (https://docs.gitlab.com/development/ee_features/#simulate-a-saas-instance)
2. Enable the use_web_based_commit_signing_enabled flag (Feature.enable(:use_web_based_commit_signing_enabled))
3. As the owner of a group try requesting the group through the REST api and check that web_based_commit_signing_enabled is in the response(For example: curl --header "PRIVATE-TOKEN: sometoken" "http://gdk.test:3000/api/v4/groups/22")
4. As the owner of a group try updating the web_based_commit_signing_enabled through the REST api (For example: curl -XPUT --header "PRIVATE-TOKEN: sometoken" "http://gdk.test:3000/api/v4/groups/22" -d "web_based_commit_signing_enabled=true")

MR acceptance checklist

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