PR template: rebase to earliest maintained version

We do not want people to try to rebase their patch on anything
prior to the earliest *maintained* version. (Currently, 10.3)

.github/pull_request_template.md

@@ -40,10 +40,11 @@ carefully describe how to test manually.
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 if the base branch for the PR is correct
+(Currently the earliest maintained branch is 10.3)
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 ## Basing the PR against the correct MariaDB version
 - [ ] *This is a new feature and the PR is based against the latest MariaDB development branch*
-- [ ] *This is a bug fix and the PR is based against the earliest branch in which the bug can be reproduced*
+- [ ] *This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced*
 
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