[NFC] Delete release branch PR template in favor of org-wide variant

`swiftlang` now has an equivalent organization-wide PR template, so we
no longer need this local one. Also, prompt folks to use this template
when targeting a release branch in the default PR template.

.github/pull_request_template.md

@@ -1,6 +1,7 @@
 <!--
-If you're opening a PR to cherry-pick a change for a release branch, use this template instead:
-https://github.com/apple/swift-docc/blob/main/.github/PULL_REQUEST_TEMPLATE/CHERRY_PICK.md
+If this pull request is targeting a release branch, please fill out the
+following form instead:
+https://github.com/swiftlang/.github/blob/main/PULL_REQUEST_TEMPLATE/release.md?plain=1
 -->
 
 Bug/issue #, if applicable: