Remove reversibility of breaking changes to GitHub Enterprise-style Jira endpoints

Summary

This issue is to remove the reversibility of breaking changes to api/v3 endpoints. See explanation on !118126 (merged) (also see #362168 (comment 1360103111)).

16.0

Based on !118126 (comment 1362169484):

• Merge !118126 (merged) with default_enabled: true.
• Enable the flag /chatops run feature set jira_dvcs_end_of_life_amnesty true --dev --staging --staging-ref --production
• Change default_enabled to false in a new MR. !119575 (merged)
• Merge !120051 (merged)

We are logging when the endpoints are hit so we can understand who are using them on GitLab.com !121095 (merged) as GitLab.com customers are part of the amnesty period #408185 (comment 1393787558).

16.6

• Remove the jira_dvcs_end_of_life_amnesty feature flag (this flag will not be rolled out)
• Clean up jira_dvcs_end_of_life_amnesty flag: /chatops run feature delete jira_dvcs_end_of_life_amnesty --dev --staging --staging-ref --production
• Delete lib/api/v3/github.rb file and related specs
• Remove all of v3 API endpoint support
• Delete Oauth::JiraDvcs::AuthorizationsController file
• Delete routes that map to Oauth::JiraDvcs::AuthorizationsController
• Remove mention of workaround from docs (probably here)
• Close the feedback issue.