[NFC] .github: 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/release.md

@@ -1,46 +0,0 @@
-<!--
-In order for a pull request to be considered for inclusion in a release branch
-(release/x.y) after it has been cut, the pull request must have the following
-information filled out in its description.
-
-To draft a pull request using this template, append '&template=release.md' to
-a pull request preview URL (the "Open a pull request" page), or use the
-following URL template:
-
-https://github.com/apple/swift/compare/main...<username>:swift:<my-branch>?expand=1&template=release.md
--->
-
-* Explanation:
-  <!--
-  A description of the change. 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?
-  -->
-* Issues:
-  <!--
-  References to issues the change resolves, if any.
-  -->
-* Original PR:
-  <!--
-  Link to the main branch version of this pull request.
-  -->
-* 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?
-  -->
-* Reviewers:
-  <!--
-  The code owners that reviewed the original PR. One or more code owners of
-  the impacted components should review the change. Technical review can be
-  delegated by a code owner or otherwise requested as deemed appropriate or
-  useful.
-  -->
-

.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.
+If this pull request is targeting a release branch, please fill out the
-For information about linking a pull request to an issue, see:
+following form:
+https://github.com/swiftlang/.github/blob/main/PULL_REQUEST_TEMPLATE/release.md?plain=1
+
+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.