[NFC] .github: Link out to Swift.org for release branch PR form

In an effort to centralize the release branch PR form and nomination
process, provide a link to the section containing the form on Swift.org
in the PR templates.

.github/PULL_REQUEST_TEMPLATE/CHERRY_PICK.md

@@ -1,6 +1,4 @@
-- **Explanation**: _A description of the issue being fixed or enhancement being made. This can be brief, but it should be clear._
-- **Scope**: _An assessment of the impact/importance of the change. For example, is the change a source-breaking language change, etc._
-- **GitHub Issue**: _The issue number if the change fixes/implements an issue/enhancement on [GitHub Issues](https://github.com/apple/swift-docc/issues)._
-- **Risk**: _What is the (specific) risk to the release for taking this change?_
-- **Testing**: _What specific testing has been done or needs to be done to further validate any impact of this change?_
-- **Reviewer**: _One or more code owners for the impacted components should review the change. Technical review can be delegated by a code owner or otherwise requested as deemed appropriate or useful._
+<!--
+Please fill out the form published at:
+https://www.swift.org/contributing#release-branch-pull-requests
+-->

.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 form
+published at:
+https://www.swift.org/contributing#release-branch-pull-requests
 -->
 
 Bug/issue #, if applicable: