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>