Support more than 0999 migrations

[chromy]

The 1000th migration (https://github.com/getsentry/sentry/pull/102006)

Suffered from a CI error:

django.db.migrations.exceptions.NodeNotFoundError: Migration sentry.1000_add_project_distribution_scope dependencies reference nonexistent parent node ('sentry', '0999_add_extrapolation_mode_to_snuba_query')
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/runner/work/sentry/sentry/tools/migrations/squash.py", line 293, in <module>
    raise SystemExit(main())
                     ~~~~^^
  File "/home/runner/work/sentry/sentry/tools/migrations/squash.py", line 282, in main
    subprocess.check_call(cmd, stdout=subprocess.DEVNULL)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '('sentry', 'django', 'makemigrations', '-n', 'squash')' returned non-zero exit status 1.

Where we could not find the parent migration: 0999_add_extrapolation_mode_to_snuba_query
This turned out to be due to the code which finds migrations expecting them to begin with a 0:

def _migrations(root: str) -> Generator[str]:
    for fname in os.listdir(root):
        if fname.startswith("0") and fname.endswith(".py"):
            yield fname

Change this to allow migrations to begin with any digit.

[runningcode]

Thanks!