Documentation/RelNotes: use .adoc extension for new security releases

When preparing the latest round of security fixes, we wrote release notes in v2.43.7, and then successively merged those up through to the various 'maint' branches. However, the 2.49 release series is the first to have commit 1f010d6bdf (doc: use .adoc extension for AsciiDoc files, 2025-01-20). This means that we should have renamed the new-but-historical release notes from *.txt to *.adoc during the merge into the 'maint-2.49' branch, but neglected to do so. Rename them accordingly to match the convention introduced by 1f010d6bdf. Since the release materials in question here were prepared before v2.50.0 was tagged, the 'maint' track for that release series is OK as is. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>