Update the changelog entry checker script to check newsfragment file extension

[anoadragon453]

This PR updates the script that the newsfragment check CI step uses in order to verify that changelog entries have one of the known newsfragment file extensions.

Inspired by failing to write a valid extension in #1465 :)

matrix-spec on  appservice-ping-spec [$!?] took 5.7s …
➜ ./scripts/check-newsfragments 
+++ Checking newsfragments for PR #
ERROR: newsfragment changelogs/client_server/newsfragments/1465.clarifications does not have one of the required file extensions for a changelog entry: .new, .feature, .clarification, .breaking, .deprecation

matrix-spec on  appservice-ping-spec [$!?] …
➜ mv changelogs/client_server/newsfragments/1465.clarification{s,}

matrix-spec on  appservice-ping-spec [$✘!?] …
➜ ./scripts/check-newsfragments                                   
+++ Checking newsfragments for PR #

Preview: https://pr1542--matrix-spec-previews.netlify.app

[turt2live]

Thanks!