jira-pr-links-improver: support Jira 9

Jira 9 has the same layout as Jira 8, so similarly to recently added
support of Jira 9 to jira_copy_summary.user.js [1], we can just use the
same code as for Jira 8.

The same `switch` statement for different versions of Jira also has a
`default:` case.  This default case assigns the value for variable
`header`, but doesn't use it and `return`s instead.  This seems as an
oversight.  I probably meant to use a `break` here to let the script
finish working even on quote-unquote "unsupported" version of Jira.
Usage of `warn` instead of `error` for the log message also supports
this hypothesis.  Let the userscript work on all versions of Jira, even
those that aren't considered "supported".

[1] 962cc3a (jira_copy_summary: support Jira 9, 2023-10-03)

jira-pr-links-improver.user.js

@@ -1,7 +1,7 @@
 // ==UserScript==
 // @name         Jira: Pull Request Link Improver
 // @namespace    https://github.com/rybak/atlassian-tweaks
-// @version      9
+// @version      10
 // @license      MIT
 // @description  Adds more convenient pull request links to Jira tickets.
 // @author       Andrei Rybak
@@ -61,12 +61,13 @@
 				header = '<div class="mod-header"><h2 class="toggle-title">Pull requests</h2></div>';
 				break;
 			case "8":
+			case "9":
 				header = '<div class="mod-header"><h4 class="toggle-title">Pull requests</h4></div>';
 				break;
 			default:
 				warn("Jira v" + jiraMajorVersion + " is not supported");
 				header = '<div class="mod-header"><h4 class="toggle-title">Pull requests</h4></div>';
-				return;
+				break;
 		}
 		$('#viewissue-devstatus-panel')
 			.prepend($(header +