[Feature flag] Enable use of Git v2.40

What

Enable the :gitaly_git_v240 feature flag, which upgrades from Git v2.39 to Git v2.40. No major risk factors are present in this Git release.

Main issue: #4982 (comment 1329905193)

Owners

• Team: Gitaly
• Most appropriate slack channel to reach out to: #g_gitaly
• Best individual to reach out to: NAME

Expectations

What release does this feature occur in first?

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Roll Out Steps

• Enable on staging
  • Is the required code deployed on staging? (howto)
  • Enable on staging (howto)
  • Add featureflagstaging to this issue (howto)
  • Test on staging (howto)
  • Verify the feature flag was used by checking Prometheus metric gitaly_feature_flag_checks_total
• Enable on production
  • Is the required code deployed on production? (howto)
  • Progressively enable in production (howto)
  • Add featureflagproduction to this issue
  • Verify the feature flag was used by checking Prometheus metric gitaly_feature_flag_checks_total
• Default-enable the feature flag (optional, only required if backwards-compatibility concerns exist)
  • Wait for release containg default-disabled feature flag.
  • Change the feature flag to default-enabled (howto)
  • Wait for release containing default-enabled feature flag.
• Remove feature flag
  • Remove the feature flag and the pre-feature-flag code (howto)
  • Remove the feature flag via chatops (howto)
  • Close this issue

Please refer to the documentation of feature flags for further information.