Default $wgCommentTableSchemaMigrationStage to MIGRATION_NEW

Default $wgCommentTableSchemaMigrationStage to MIGRATION_NEW

WRITE_NEW mode seems to be working well on Wikimedia sites. Let's change
the default to MIGRATION_NEW so existing installs and Wikimedia CI will
start using the new code rather than the old.

This also fixes some unit tests that were broken with MIGRATION_NEW, and
updates some that were forcing MIGRATION_OLD to force MIGRATION_NEW
instead.

Bug: T166733
Change-Id: I7bf4ad0105dd1f6cc49eba3ddcb7a51badcd5ed3
Depends-On: I30f7cdcc3875f3f7af116c1e41e88f62ab9e91d0