[FIX] github core: target latest stable branch

Part-of: odoo/odoo#77872

CONTRIBUTING.md

@@ -8,7 +8,7 @@ TL;DR
 * If you [make a pull request](https://github.com/odoo/odoo/wiki/Contributing#making-pull-requests),
   do not create an issue! Use the PR description for that
 * Issues are handled with a much lower priority than pull requests
-* Use this [template](https://github.com/odoo/odoo/tree/14.0/.github/ISSUE_TEMPLATE.md)
+* Use this [template](https://github.com/odoo/odoo/tree/15.0/.github/ISSUE_TEMPLATE.md)
   when reporting issues. Please search for duplicates first!
 * Pull requests must be made against the [correct version](https://github.com/odoo/odoo/wiki/Contributing#against-which-version-should-i-submit-a-patch)
 * There are restrictions on the kind of [changes allowed in stable series](https://github.com/odoo/odoo/wiki/Contributing#what-does-stable-mean)