Remove reversibility of Jira endpoints

What does this MR do and why?

This MR is removing the reversibility of breaking changes to api/v3 endpoints.

• Remove the jira_dvcs_end_of_life_amnesty feature flag (this flag will not be rolled out)
• 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

Screenshots or screen recordings

N/A

How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #408148 (closed)