Fix naming of unstable status.

Unstable build can have several reasons:
- failing tests
- uncovered lines
- SpotBugs warnings

src/main/resources/org/jenkinsci/plugins/github_branch_source/Messages.properties

@@ -13,7 +13,7 @@ ForkPullRequestDiscoveryTrait.nobodyDisplayName=Nobody
 GitHubBranchFilter.DisplayName=GitHub Branch Jobs Only
 
 GitHubBuildStatusNotification.CommitStatus.Good=This commit looks good
-GitHubBuildStatusNotification.CommitStatus.Unstable=This commit has test failures
+GitHubBuildStatusNotification.CommitStatus.Unstable=This commit has failing tests or missing quality gates
 GitHubBuildStatusNotification.CommitStatus.Failure=This commit cannot be built
 GitHubBuildStatusNotification.CommitStatus.Aborted=The build of this commit was aborted
 GitHubBuildStatusNotification.CommitStatus.Other=Something is wrong with the build of this commit