docker: Update to latest stable MySQL, PostgreSQL

docker: Update to latest stable MySQL, PostgreSQL

MySQL 9.7 changed the default gtid_mode from OFF to ON (since 9.5). When
the MySQL Docker container initialises, the init scripts run with
GTID=ON, leaving non-empty GTID_EXECUTED in the binary log even though
the final server starts with --gtid-mode=OFF.

This causes two problems:

1. mysqldump always includes SET @@GLOBAL.GTID_PURGED when cloning
   test databases for parallel tests. Restoring this requires
   SYSTEM_VARIABLES_ADMIN, which was not granted.

2. mysqldump requires PROCESS to dump tablespace information. While
   PROCESS was being granted, SYSTEM_VARIABLES_ADMIN was missing
   from the grants, causing clone failures (exit code 5) for Django
   6.0's stricter parallel test runner.

Fix both issues by:

- Running RESET BINARY LOGS AND GTIDS before granting privileges,
  which clears the GTID state left by the init phase so mysqldump
  no longer emits GTID_PURGED statements.

- Adding SYSTEM_VARIABLES_ADMIN to the privilege grants so that if
  any GTID state is present (e.g. in future MySQL versions), the
  restore can set GTID_PURGED.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>