ceph-backport.sh: run on macOS as well as Linux

The script relies on GNU bash >= 4 (associative arrays, ${var,,}) and GNU
getopt (--long options). macOS ships bash 3.2 and BSD getopt, so the script
died with a cryptic "declare: -A: invalid option" before it could explain
itself.

Add a small portability guard near the top that:

  * re-execs the script under a bash >= 4 when the running interpreter is
    too old, searching $BASH_OVERRIDE, the common Homebrew locations, and
    $PATH; and
  * resolves getopt through a $GETOPT override (default: getopt) and confirms
    it is GNU/enhanced getopt via "getopt --test".

When the required tools are absent the script now prints an actionable hint
("brew install bash gnu-getopt") instead of a bash error. The Linux path is
unchanged: bash there is already >= 4 and getopt is GNU, so neither the
re-exec nor any override triggers.

Also add a PendingReleaseNotes entry.

Assisted-by: Claude Opus 4.8
Signed-off-by: Anthony D'Atri <anthonyeleven@users.noreply.github.com>