[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.

.github/pull_request_template.md

@@ -1,12 +1,17 @@
-<!-- What's in this pull request? -->
-Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.
-
 <!--
-If this pull request resolves any GitHub issues, link them.
-For information about linking a pull request to an issue, see:
+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
+
+Otherwise, replace this comment with a description of your changes and
+rationale. Provide links to external references/discussions if appropriate.
+If this pull request resolves any GitHub issues, link them like so:
+
+  Resolves <link to issue>, resolves <link to another issue>.
+
+For more information about linking a pull request to an issue, see:
 https://docs.github.com/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue
 -->
-Resolves #NNNNN, fix apple/llvm-project#MMMMM.
 
 <!--
 Before merging this pull request, you must run the Swift continuous integration tests.