[JENKINS-33273] Jenkinsfile from SCMFileSystem

[jglick]

Downstream of jenkinsci/workflow-multibranch-plugin#47, jenkinsci/workflow-cps-plugin#97, jenkinsci/git-plugin#467, and jenkinsci/pipeline-github-lib-plugin#1.

SCMFile access was broken for pull requests: head PRs did not correctly get the actual saved head revision; and merge PRs did not work at all:

ERROR: Could not do lightweight checkout, falling back to heavyweight
java.io.FileNotFoundException: 404: Not Found

	at org.kohsuke.github.Requester.handleApiError(Requester.java:666)
	at org.kohsuke.github.Requester.asStream(Requester.java:320)
	at org.kohsuke.github.GHContent.read(GHContent.java:119)
	at org.jenkinsci.plugins.github_branch_source.GitHubSCMFile.content(GitHubSCMFile.java:170)
	at jenkins.scm.api.SCMFile.contentAsString(SCMFile.java:338)
	at org.jenkinsci.plugins.workflow.multibranch.SCMBinder.create(SCMBinder.java:88)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:215)
	at hudson.model.ResourceController.execute(ResourceController.java:98)
	at hudson.model.Executor.run(Executor.java:404)
Caused by: java.io.FileNotFoundException: https://raw.githubusercontent.com/cloudbeers/multibranch-demo/refs/pull/14/merge/Jenkinsfile
	at com.squareup.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:243)
	at com.squareup.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
	at com.squareup.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:25)
	at org.kohsuke.github.Requester.asStream(Requester.java:318)
	... 7 more

I am disabling the merge PR support since I doubt it can be made to work. (Yes GitHub does offer some kind of remote support for PR merge results, but it is rather broken and we do not use it.)

@reviewbybees esp. @stephenc

[ghost]

This pull request originates from a CloudBees employee. At CloudBees, we require that all pull requests be reviewed by other CloudBees employees before we seek to have the change accepted. If you want to learn more about our process please see this explanation.