feat(workflows): close individual PRs when combined PR is merged (#1531)

Adds `Closes` keyword before the successfully merged PR numbers so that GitHub automatically closes the individual PRs when the combined PR is merged.

.github/workflows/combine-prs.yml

@@ -96,7 +96,7 @@ jobs:
                 }
                 if (statusOK) {
                   console.log('Adding branch to array: ' + branch);
-                  const prString = '#' + pull['number'] + ' ' + pull['title'];
+                  const prString = 'Closes #' + pull['number'] + ' ' + pull['title'];
                   branchesAndPRStrings.push({ branch, prString });
                   baseBranch = pull['base']['ref'];
                   baseBranchSHA = pull['base']['sha'];