docs(ISSUE_TEMPLATE.md): add Opera to browser list

Opera is indirectly supported, although not explicitly tested on (see
[here](https://docs.angularjs.org/misc/faq#what-browsers-does-angularjs-work-with-)
for more info), so it doesn't hurt adding it to the list of browsers.

Closes #16801

Author: julkwel

.github/ISSUE_TEMPLATE.md

@@ -30,7 +30,7 @@ https://plnkr.co or similar (you can use this template as a starting point: http
 <!-- Check whether this is still an issue in the most recent stable or in the snapshot AngularJS
 version (https://code.angularjs.org/snapshot/) -->
 
-**Browser:** [all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
+**Browser:** [all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView | Opera XX ]
 <!-- All browsers where this could be reproduced (and Operating System if relevant) -->
 
 **Anything else:**