Update postgis version in CI to latest

[tim-schilling]

We need to update the postgis version from postgis/postgis:14-3.1 to the latest (see their Docker page for the latest versions)

This is because we're getting the following error in CI:

py313-djmain-psycopg3: commands[0]> python -b -W always -m coverage run -m django test -v2 tests
  Creating test database for alias 'default' ('test_debug_toolbar')...
  /home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/postgresql/base.py:513: RuntimeWarning: Normally Django will use a connection to the 'postgres' database to avoid running initialization queries against the production database when it's not needed (for example, when running tests). Django was unable to create a connection to the 'postgres' database and will use the first PostgreSQL database instead.
    warnings.warn(
  Traceback (most recent call last):
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/postgresql/base.py", line 508, in _nodb_cursor
      with super()._nodb_cursor() as cursor:
           ~~~~~~~~~~~~~~~~~~~~^^
    File "/opt/hostedtoolcache/Python/3.13.7/x64/lib/python3.13/contextlib.py", line 141, in __enter__
      return next(self.gen)
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 712, in _nodb_cursor
      with conn.cursor() as cursor:
           ~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/utils/asyncio.py", line 26, in inner
      return func(*args, **kwargs)
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 320, in cursor
      return self._cursor()
             ~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 296, in _cursor
      self.ensure_connection()
      ~~~~~~~~~~~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/utils/asyncio.py", line 26, in inner
      return func(*args, **kwargs)
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 279, in ensure_connection
      self.connect()
      ~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/utils/asyncio.py", line 26, in inner
      return func(*args, **kwargs)
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 258, in connect
      self.init_connection_state()
      ~~~~~~~~~~~~~~~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/postgresql/base.py", line 399, in init_connection_state
      super().init_connection_state()
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 225, in init_connection_state
      self.check_database_version_supported()
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
    File "/home/runner/work/django-debug-toolbar/django-debug-toolbar/.tox/py313-djmain-psycopg3/lib/python3.13/site-packages/django/db/backends/base/base.py", line 201, in check_database_version_supported
      raise NotSupportedError(
      ...<2 lines>...
      )
  django.db.utils.NotSupportedError: PostgreSQL 15 or later is required (found 14.1).