release: Link to the changelog in the Github Release.

Release notes in GitHub have significant-newlines, which makes our
canonical `changelog.md` content look not-great -- and the relative
links are also broken, as they have the wrong relative path.

Switch to linking to the canonical release notes on ReadTheDocs, now
that those have proper anchors.

(cherry picked from commit c1008b5b12)

tools/release

@@ -112,6 +112,7 @@ gh auth status \
     || fail "Not authenticated to github"
 
 # Extract the changelog, print it
+changelog_anchor="zulip-server-${version//./-}"
 changelog=$(VERSION="$version" perl -nle '$v=quotemeta($ENV{VERSION}); print if $rc = /^### $v/ .. /^#{1,3} (?!$v)/ and $rc !~ /E0/' docs/overview/changelog.md)
 echo "$changelog"
 
@@ -145,6 +146,6 @@ if [ -n "$is_prerelease" ]; then
 fi
 gh release create "$version" \
     --title "Zulip Server $version" \
-    --notes-file <(echo "$changelog") \
+    --notes "[Complete release notes](https://zulip.readthedocs.io/en/latest/overview/changelog.html#${changelog_anchor})" \
     "${params[@]}" \
     "$TARBALL"