Part of Stable Releases.

Overview

See #12458 for context.

In addition to bisecting the current release script to support stable and mainline, we need to add a new release script to migrate the stable label to the latest mainline release. This happens after 2 weeks of fielding the mainline release to the community as our "latest" edge release.

Requirements

Example usage, promoting 2.9.1 to stable from 2.8.3:
promote_stable.sh v2.9.1

• Accepts the desired version tag to be upgraded
• Migrates latest github tag to target version
• Appends "(Stable)" to the target version's release name
• Removes "(Stable)" from previous versions's release name
• CODER_MAINLINE_VERSION in install.sh does not need updating

When this action occurs, mainline and stable will point to the same version for 2 weeks. This is acceptable.