Add suggestion to PR template to encourage contributors to use a GitHub link their PR to issue (#21624)

Author: Mattias-Sehlstedt

.github/PULL_REQUEST_TEMPLATE.md

@@ -18,4 +18,5 @@
   You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example `./bin/generate-samples.sh bin/configs/java*`. 
   IMPORTANT: Do **NOT** purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
 - [ ] File the PR against the [correct branch](https://github.com/OpenAPITools/openapi-generator/wiki/Git-Branches): `master` (upcoming `7.x.0` minor release - breaking changes with fallbacks), `8.0.x` (breaking changes without fallbacks)
+- [ ] If your PR solves a reported issue, reference it using [GitHub's linking syntax](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) (e.g., having `"fixes #123"` present in the PR description)
 - [ ] If your PR is targeting a particular programming language, @mention the [technical committee](https://github.com/openapitools/openapi-generator/#62---openapi-generator-technical-committee) members, so they are more likely to review the pull request.