CI: github: set ulimit -n to a greater value

Set ulimit -n to 65536 to limit less the maxconn computation.

Could be backported at least to 2.5.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index a7cdcc5..8c13d13 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -136,7 +136,7 @@
       run: |
         # This is required for macOS which does not actually allow to increase
         # the '-n' soft limit to the hard limit, thus failing to run.
-        ulimit -n 5000
+        ulimit -n 65536
         make reg-tests VTEST_PROGRAM=../vtest/vtest REGTESTS_TYPES=default,bug,devel
     - name: Show VTest results
       if: ${{ failure() && steps.vtest.outcome == 'failure' }}