Ensure docs gen inserts at correct place in file (#18250)

The regex for finding the latest release (as a fallback when current is not in
the file) had a bug. This caused the generated docs to be inserted at the wrong
place in the file (the end of the file) instead of the top. This commit fixes
the logic such that we find the last release when the current is not found.

(cherry picked from commit a3228e2)

Author: donoghuc

tools/release/generate_release_notes_md.rb

@@ -42,7 +42,7 @@
 
 coming_tag_index = release_notes.find_index {|line| line.match(/^## #{current_release} \[logstash-#{current_release}-release-notes\]$/) }
 coming_tag_index += 1 if coming_tag_index
-release_notes_entry_index = coming_tag_index || release_notes.find_index {|line| line.match(/\[logstash-\d+-release-notes\]$/) }
+release_notes_entry_index = coming_tag_index || release_notes.find_index {|line| line.match(/^## .*\[logstash-.*-release-notes\]$/) }
 
 unless coming_tag_index
   report << "## #{current_release} [logstash-#{current_release}-release-notes]\n\n"