Add addendum to private val parameter variance error message

[mbovel]

Implements addendum discussed for #22620.

[mbovel]

We should also disable the -rewrite for private[this] class parameters. Unfortunately, we don't know if we are in a parameters list at the point where we issue the rewrite. We'd probably need to thread that information there.

scala3/compiler/src/dotty/tools/dotc/parsing/Parsers.scala

Lines 3427 to 3435 in e6c474d

	if mods1.is(Local) then
	report.errorOrMigrationWarning(
	em"""Ignoring [this] qualifier.
	|This syntax will be deprecated in the future; it should be dropped.
	|See: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html${rewriteNotice(`3.4-migration`)}""",
	in.sourcePos(),
	MigrationVersion.RemoveThisQualifier)
	if MigrationVersion.RemoveThisQualifier.needsPatch then
	patch(source, Span(startOffset, in.lastOffset), "")

[mbovel]

We should also disable the -rewrite for private[this] class parameters.

Discussed during today's compiler meeting; we won't fix the rewrite as it would require too many changes.