[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.

Author: AnthonyLatsis

.github/PULL_REQUEST_TEMPLATE/release.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
+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.
 
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