feat: rework notification table (#9926)

This change is motivated by 5e300a2a87

- Drop the `updated_by` and `commit_id` column, they are unused and have a index for no reason.
- Drop the index on `status` and `created_unix` and make a index on `(user_id, status)`.

## Test
1. Run migration.
2. Confirm the migration succeeds.
3. Check that `notification` table has the correct indexes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9926
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

templates/swagger/v1_json.tmpl

@@ -2192,7 +2192,6 @@
               "enum": [
                 "issue",
                 "pull",
-                "commit",
                 "repository"
               ],
               "type": "string"
@@ -13520,7 +13519,6 @@
               "enum": [
                 "issue",
                 "pull",
-                "commit",
                 "repository"
               ],
               "type": "string"