Depend on git client plugin 6.1.0 (#1672)

Git client plugin 6.1.0 uses Jakarta EE 9 and requires Java 17.  Better to
assure that users of git plugin 5.6.0 also have the matching version
of git client plugin for Jakarta EE 9.  Reduces the version testing
complications and lets me focus on single versions of each plugin in
the 2.479 testing.

pom.xml

@@ -106,6 +106,8 @@
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>git-client</artifactId>
+      <!-- TODO: Remove when release is available in plugin BOM -->
+      <version>6.1.0</version>
     </dependency>
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>