Bump bom-2.263.x from 20 to 21 (#72)

Bumps [bom-2.263.x](https://github.com/jenkinsci/pom) from 20 to 21.
- [Release notes](https://github.com/jenkinsci/pom/releases)
- [Changelog](https://github.com/jenkinsci/pom/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jenkinsci/pom/commits)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pom.xml

@@ -53,7 +53,7 @@
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
         <artifactId>bom-2.263.x</artifactId>
-        <version>20</version>
+        <version>21</version>
         <scope>import</scope>
         <type>pom</type>
       </dependency>