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

mgol/angular.js,angular/angular.js,ze-pequeno/angular.js,eistrati/angular.js,reneploetz/angular.js,gkalpak/angular.js,eistrati/angular.js,ze-pequeno/angular.js,ze-pequeno/angular.js,gkalpak/angular.js,mgol/angular.js,mosoft521/angular.js,angular/angular.js,mgol/angular.js,reneploetz/angular.js,eistrati/angular.js,mosoft521/angular.js,ze-pequeno/angular.js,angular/angular.js,angular/angular.js,gkalpak/angular.js,mgol/angular.js,gkalpak/angular.js,reneploetz/angular.js,eistrati/angular.js,reneploetz/angular.js,mosoft521/angular.js,mosoft521/angular.js