Cells: Add composite index index_ssh_signatures_on_project_id_and_commit_sha for ssh_signatures table

What does this MR do and why?

As part of Cells, we need to ensure that the unique index by commit_sha is still unique for ssh_signatures table.

In this MR, we drop index_ssh_signatures_on_commit_sha and redundant index_ssh_signatures_on_project_id indexes in favor of index_ssh_signatures_on_project_id_and_commit_sha since the same commit SHA cannot exist across different projects in any meaningful way.

Database

Before migration

                                             Table "public.ssh_signatures"
         Column         |           Type           | Collation | Nullable |                  Default
------------------------+--------------------------+-----------+----------+--------------------------------------------
 id                     | bigint                   |           | not null | nextval('ssh_signatures_id_seq'::regclass)
 created_at             | timestamp with time zone |           | not null |
 updated_at             | timestamp with time zone |           | not null |
 project_id             | bigint                   |           | not null |
 key_id                 | bigint                   |           |          |
 verification_status    | smallint                 |           | not null | 0
 commit_sha             | bytea                    |           | not null |
 user_id                | bigint                   |           |          |
 key_fingerprint_sha256 | bytea                    |           |          |
 committer_email        | text                     |           |          |
Indexes:
    "ssh_signatures_pkey" PRIMARY KEY, btree (id)
    "index_ssh_signatures_on_commit_sha" UNIQUE, btree (commit_sha)
    "index_ssh_signatures_on_key_id" btree (key_id)
    "index_ssh_signatures_on_project_id" btree (project_id)
    "index_ssh_signatures_on_user_id" btree (user_id)
Check constraints:
    "check_73776e38f9" CHECK (char_length(committer_email) <= 255)
Foreign-key constraints:
    "fk_0c83baaa5f" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    "fk_7d2f93996c" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    "fk_aa1efbe865" FOREIGN KEY (key_id) REFERENCES keys(id) ON DELETE SET NULL

After migration

                                             Table "public.ssh_signatures"
         Column         |           Type           | Collation | Nullable |                  Default
------------------------+--------------------------+-----------+----------+--------------------------------------------
 id                     | bigint                   |           | not null | nextval('ssh_signatures_id_seq'::regclass)
 created_at             | timestamp with time zone |           | not null |
 updated_at             | timestamp with time zone |           | not null |
 project_id             | bigint                   |           | not null |
 key_id                 | bigint                   |           |          |
 verification_status    | smallint                 |           | not null | 0
 commit_sha             | bytea                    |           | not null |
 user_id                | bigint                   |           |          |
 key_fingerprint_sha256 | bytea                    |           |          |
 committer_email        | text                     |           |          |
Indexes:
    "ssh_signatures_pkey" PRIMARY KEY, btree (id)
    "index_ssh_signatures_on_key_id" btree (key_id)
    "index_ssh_signatures_on_project_id_and_commit_sha" UNIQUE, btree (project_id, commit_sha)
    "index_ssh_signatures_on_user_id" btree (user_id)
Check constraints:
    "check_73776e38f9" CHECK (char_length(committer_email) <= 255)
Foreign-key constraints:
    "fk_0c83baaa5f" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    "fk_7d2f93996c" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    "fk_aa1efbe865" FOREIGN KEY (key_id) REFERENCES keys(id) ON DELETE SET NULL

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Related to #562085 (closed)