Re add index_ssh_signatures_on_commit_sha index

What does this MR do and why?

The index index_ssh_signatures_on_commit_sha was removed in Reintroduce composite index_ssh_signatures_on_p... (!216058 - merged) but is needed for queries that filter by commit_sha.

Local Migration Output

Up

bundle exec rake db:migrate:main VERSION=20260106095223
main: == [advisory_lock_connection] object_id: 132340, pg_backend_pid: 21801
main: == 20260106095223 ReAddIndexSshSignaturesOnCommitSha: migrating ===============
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0289s
main: -- index_exists?(:ssh_signatures, :commit_sha, {:unique=>true, :name=>"index_ssh_signatures_on_commit_sha", :algorithm=>:concurrently})
main:    -> 0.0019s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:ssh_signatures, :commit_sha, {:unique=>true, :name=>"index_ssh_signatures_on_commit_sha", :algorithm=>:concurrently})
main:    -> 0.0032s
main: -- execute("RESET statement_timeout")
main:    -> 0.0006s
main: == 20260106095223 ReAddIndexSshSignaturesOnCommitSha: migrated (0.0697s) ======

Down

bundle exec rake db:migrate:down:main VERSION=
20260106095223
main: == [advisory_lock_connection] object_id: 131960, pg_backend_pid: 26444
main: == 20260106095223 ReAddIndexSshSignaturesOnCommitSha: reverting ===============
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0290s
main: -- indexes(:ssh_signatures)
main:    -> 0.0018s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:ssh_signatures, {:algorithm=>:concurrently, :name=>"index_ssh_signatures_on_commit_sha"})
main:    -> 0.0018s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20260106095223 ReAddIndexSshSignaturesOnCommitSha: reverted (0.0664s) ======

main: == [advisory_lock_connection] object_id: 131960, pg_backend_pid: 26444

References

Followup from this comment: !216058 (comment 2985644705)

Incident follow-up: gitlab-com/gl-infra/production-engineering#28113 (closed)

Incident caused by the index removal: gitlab-com/gl-infra/production#21046 (closed)

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