chore(version): bump kmptoolkit.version to 3.5.21 after release of 3.5.20 (#169)

Auto-opened by mbl-actionhub-bump-version. Sets kmptoolkit.version=3.5.21
so the next release run has a valid sanity-check value
(3.5.21 > 3.5.20 on Maven Central).

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

gradle.properties

@@ -5,7 +5,7 @@
 # After this release, mbl-actionhub-bump-version auto-opens a PR bumping
 # this value to the next patch — manual edits should be rare from now on.
 # =============================================================================
-kmptoolkit.version=3.5.20
+kmptoolkit.version=3.5.21
 
 # Maven Central Publishing (vanniktech 0.36.0 reads these from gradle.properties)
 SONATYPE_HOST=CENTRAL_PORTAL