docs(issues): encourage the use of conventional commits

It is not enforced but it is widely spread and encouraging its use may
lead to more uniform issue titles overall.

https://www.conventionalcommits.org/en/v1.0.0/

.gitea/issue_template/bug-report-ui.yaml

@@ -1,6 +1,6 @@
 name: 🦋 Bug Report (web interface / frontend)
 description: Something doesn't look quite as it should?  Report it here!
-title: "[BUG] "
+title: "bug: "
 labels: ["bug/new-report", "forgejo/ui"]
 body:
 - type: markdown

.gitea/issue_template/bug-report.yaml

@@ -1,6 +1,6 @@
 name: 🐛 Bug Report (server / backend)
 description: Found something you weren't expecting? Report it here!
-title: "[BUG] "
+title: "bug: "
 labels: bug/new-report
 body:
 - type: markdown

.gitea/issue_template/feature-request.yaml

@@ -1,6 +1,6 @@
 name: 💡 Feature Request
 description: Got an idea for a feature that Forgejo doesn't have yet? Suggest it here!
-title: "[FEAT] "
+title: "feat: "
 labels: ["enhancement/feature"]
 body:
 - type: markdown