fix: nil: Do not hardcode release date in man pages

The util/meson-dist-package.sh script hardcodes the date it is run on
into the man pages it creates in the dist tarball.  This causes pkgdiff
to report discrepancies if the util/release-tarball-comparison.sh script
is run on a different day than the one the dist tarball was generated
on.

Fix by using the exact same solution as in BIND 9.20: generating the man
page stubs with a @RELEASE_DATE@ placeholder instead of a specific date
and only replacing that placeholder with a specific date during the
build process.

Closes #5412

Merge branch '5412-do-not-hardcode-release-date-in-man-pages' into 'main'

See merge request isc-projects/bind9!10716