config: Introduce :list_commits feature flag

Introduce a new :list_commits feature flag which will be used to switch
from the `ListNewCommits()` and `CommitsBetween()` RPCs to the new
`ListCommits()` RPC.

config/feature_flags/development/list_commits.yml

+---
+name: list_commits
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65468
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/335208
+milestone: '14.1'
+type: development
+group: group::gitaly
+default_enabled: false