Change default branch to updated scip-ruby version

[emidoots]

Once these three PRs are merged: #261, #262, #263

[linear]

OSS-24223

[emidoots]

1. All three PRs were reviewed/merged via CLI (to avoid squash merge), the last commit in update to latest Sorbet (change 3 of 3): additional fixes #263 is db9f39e - that will be our new default branch.
2. I have created a new master branch (not scip-ruby/master) which is set to db9f39e
3. The default branch has changed from scip-ruby/master to master.
4. I have renamed scip-ruby/master (old branch) to legacy, which is commit eaa63a4 https://github.com/sourcegraph/scip-ruby/tree/legacy
5. I have deleted the now-unused updated branch.