Add security note to issue template (#6281)

.github/issue_template.md

@@ -1,7 +1,9 @@
+<!-- NOTE: If your issue is a security concern, please send an email to security@gitea.io instead of opening a public issue -->
+
 <!--
-    1. Please speak English, this is the language all of us can speak and write.
+    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/NsatcWJ) or forum (https://discourse.gitea.io).
+       server (https://discord.gg/gitea) or forum (https://discourse.gitea.io).
     3. Please take a moment to check that your issue doesn't already exist.
     4. Please give all relevant information below for bug reports, because 
        incomplete details will be handled as an invalid report.