Update demo site location from try.gitea.io -> demo.gitea.com (#31054) (#31146)

Backport #31054 by @lunny

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.gitea/issue_template.md

@@ -3,7 +3,7 @@
 <!--
     1. Please speak English, this is the language all maintainers can speak and write.
     2. Please ask questions or configuration/deploy problems on our Discord
-       server (https://discord.gg/gitea) or forum (https://discourse.gitea.io).
+       server (https://discord.gg/gitea) or forum (https://forum.gitea.com).
     3. Please take a moment to check that your issue doesn't already exist.
     4. Make sure it's not mentioned in the FAQ (https://docs.gitea.com/help/faq)
     5. Please give all relevant information below for bug reports, because
@@ -21,7 +21,7 @@
   - [ ] MySQL
   - [ ] MSSQL
   - [ ] SQLite
-- Can you reproduce the bug at https://try.gitea.io:
+- Can you reproduce the bug at https://demo.gitea.com:
   - [ ] Yes (provide example URL)
   - [ ] No
 - Log gist: