difflib could return an empty string when the only difference is in the
line endings of the strings being compared, which currently leads to a
"silent" error (because the empty diff is output).

Instead, let's explicitly clarify that that is what happened.

This will also allow using mdoc-parser which no longer works with java8.

Now that scalameta supports jdk21, and scalafmt uses mdoc-parser, let's
start validating the formatter with jdk21 in pre-commit checks.