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

In an effort to centralize the release branch PR form and release
process info in general, remove any instances of such content and
link out to the dedicated section on Swift.org instead.

Author: AnthonyLatsis

.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: