Remove legacy build directory caching

Description

In !1319 (merged) we added back the old way of how we cache the build directory for Docker since it causes regressions like #4174 (closed). However, the old behavior is incorrect and we should remove this earlier then %13.0 becuase %13.0 is a little long (1 year from now as of the time of writing)

Change Impact and Benefits

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Proposal

1. %11.11 Update the feature flag documentation to specify %12.3
2. In %11.11 %12.0 release post announces that this is going to change.
3. In %12.0 update documentation educating users that they should never us build_dir for their own stuff, but should always be outside of that directory #4436 (closed)
4. In %13.0 remove the whole thing

To do List

• Publish a blog post about the upcoming change, with a timeline and a simple way to recognize who is affected, and who is not.
• Ask to schedule tweets about the blog post in the #twitter Slack channel.
• Ask to reach out customers that may be affected by the change in the #customer-success and #sales Slack channels.
• Mention the problems customers may report and how to address them in the #support_self-managed and #support_gitlab-com Slack channels