.gitlab: update issue template to reflect new default branch

Signed-off-by: Aster Boese <asterboese@mailbox.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8183>

.gitlab/issue_templates/Default.md

@@ -26,7 +26,7 @@
 <!--
 	Replace space inside `[ ]` with `x` to fill the checkbox
 -->
-* [ ] edge (`master` branch)
+* [ ] edge (`main` branch)
 * [ ] `v25.12`
 
 * [ ] I confirm that the issue still is present after running `sudo apk upgrade -a`