build: update gnulib submodule to latest

build: update gnulib submodule to latest

This fixes failures in "very-expensive" tests on FTS with many
directory entries:
  FAIL: tests/rm/ext3-perf
  FAIL: tests/rm/many-dir-entries-vs-OOM

The following shows the problem in the former of the above tests:
  $ mkdir d && seq 400000 | env -C d xargs touch )
  $ rm -rf d
  rm: traversal failed: d: Operation not supported

Gnulib commit 3f0950f65abb (2023-04-26) introduced this regression
which was fixed again with gnulib commit d4d8abb39eb0.

See discussion in
<https://lists.gnu.org/r/bug-gnulib/2023-05/msg00040.html>

* bootstrap.conf (gnulib_modules): Change "year2038-required" to
"year2038-recommended"; the module has been replaced.
* gnulib: Update to latest.
* tests/init.sh: Likewise.