Upgraded Git &  Git Client to latest versions.

Nightly builds failed with this error...

The recommended git tool is: NONE
FATAL: com/trilead/ssh2/ServerHostKeyVerifier has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
java.lang.UnsupportedClassVersionError: com/trilead/ssh2/ServerHostKeyVerifier has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at org.apache.tools.ant.AntClassLoader.defineClassFromData(AntClassLoader.java:1172)
    at org.apache.tools.ant.AntClassLoader.getClassFromStream(AntClassLoader.java:1340)
    at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1392)
    at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1357)
    at jenkins.ClassLoaderReflectionToolkit.loadClass(ClassLoaderReflectionToolkit.java:148)
    at hudson.ClassicPluginStrategy$DependencyClassLoader.findClass(ClassicPluginStrategy.java:650)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at org.apache.tools.ant.AntClassLoader.findBaseClass(AntClassLoader.java:1421)
    at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1108)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at org.apache.tools.ant.AntClassLoader.defineClassFromData(AntClassLoader.java:1172)
    at org.apache.tools.ant.AntClassLoader.getClassFromStream(AntClassLoader.java:1340)
    at org.apache.tools.ant.AntClassLoader.findClassInComponents(AntClassLoader.java:1392)
    at org.apache.tools.ant.AntClassLoader.findClass(AntClassLoader.java:1357)
    at org.apache.tools.ant.AntClassLoader.loadClass(AntClassLoader.java:1112)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at org.jenkinsci.plugins.gitclient.verifier.KnownHostsFileVerificationStrategy.getVerifier(KnownHostsFileVerificationStrategy.java:17)
    at org.jenkinsci.plugins.gitclient.verifier.KnownHostsFileVerificationStrategy.getVerifier(KnownHostsFileVerificationStrategy.java:8)
    at org.jenkinsci.plugins.gitclient.Git.getClient(Git.java:137)
    at hudson.plugins.git.GitSCM.createClient(GitSCM.java:916)
    at hudson.plugins.git.GitSCM.createClient(GitSCM.java:847)
    at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1297)
    at hudson.scm.SCM.checkout(SCM.java:540)
    at hudson.model.AbstractProject.checkout(AbstractProject.java:1217)
    at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:647)
    at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:85)
    at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:519)
    at hudson.model.Run.execute(Run.java:1897)
    at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:44)
    at hudson.model.ResourceController.execute(ResourceController.java:101)
    at hudson.model.Executor.run(Executor.java:442)
Finished: FAILURE

I noticed when configuring SCM Git repository URL's, that form validation failed when entering valid Git url,

So downgraded plugins until builds succeeded with Git plugin 4.11.4 & Git Client 3.11.0