Log spam `Pull request … : Not mergeable` from `BuildStatusChecksPublisher$ChecksGraphListener`

### Jenkins and plugins versions report

`github-checks` 1.0.13 running on Jenkins 2.319.3 (as CloudBees CI), with `checks-api` 1.7.2 & `github-branch-source` 2.11.4.

### What Operating System are you using (both controller, and any agents involved in the problem)?

Linux

### Reproduction steps

Not known exactly; presumably some pull requests with merge conflicts, on a multibranch pipeline using PR-merge strategy.

### Expected Results

Nothing in the controller log, or at most a single-line notice.

### Actual Results

Numerous stack traces of the form

```
WARNING o.j.p.w.cps.CpsFlowExecution#notifyListeners
hudson.AbortException: Pull request … : Not mergeable at …+… (NOT_MERGEABLE)
at org.jenkinsci.plugins.github_branch_source.PullRequestSCMRevision.validateMergeHash(PullRequestSCMRevision.java:116)
at org.jenkinsci.plugins.github_branch_source.GitHubSCMSource.retrieve(GitHubSCMSource.java:1697)
at jenkins.scm.api.SCMSource.fetch(SCMSource.java:582)
at io.jenkins.plugins.checks.github.SCMFacade.findRevision(SCMFacade.java:156)
Caused: java.lang.IllegalStateException: Could not fetch revision from repository: … and branch: PR-…
at io.jenkins.plugins.checks.github.SCMFacade.findRevision(SCMFacade.java:159)
at io.jenkins.plugins.checks.github.GitHubSCMSourceChecksContext.resolveHeadSha(GitHubSCMSourceChecksContext.java:131)
at io.jenkins.plugins.checks.github.GitHubSCMSourceChecksContext.(GitHubSCMSourceChecksContext.java:46)
at io.jenkins.plugins.checks.github.GitHubSCMSourceChecksContext.fromRun(GitHubSCMSourceChecksContext.java:24)
at io.jenkins.plugins.checks.github.GitHubChecksPublisherFactory.createPublisher(GitHubChecksPublisherFactory.java:48)
at io.jenkins.plugins.checks.api.ChecksPublisherFactory.lambda$fromRun$0(ChecksPublisherFactory.java:89)
at …
at java.util.stream.ReferencePipeline.findFirst(ReferencePipeline.java:531)
at io.jenkins.plugins.checks.api.ChecksPublisherFactory.fromRun(ChecksPublisherFactory.java:92)
at io.jenkins.plugins.checks.api.ChecksPublisherFactory.fromRun(ChecksPublisherFactory.java:69)
at io.jenkins.plugins.checks.status.BuildStatusChecksPublisher$ChecksGraphListener.lambda$onNewHead$0(BuildStatusChecksPublisher.java:257)
at java.util.Optional.ifPresent(Optional.java:159)
at io.jenkins.plugins.checks.status.BuildStatusChecksPublisher$ChecksGraphListener.onNewHead(BuildStatusChecksPublisher.java:257)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.notifyListeners(CpsFlowExecution.java:1473)
at …
```

### Anything else?

_No response_