Fix supported branches in PR template backports section (#242)

Signed-off-by: eProsima <[email protected]>

Author: JesusPoderoso

.github/pull_request_template.md

@@ -17,7 +17,7 @@
     In case of bug fixes, please provide the list of supported branches where this fix should be also merged.
     Please uncomment following line, adjusting the corresponding target branches for the backport.
 -->
-<!-- @Mergifyio backport 2.2.x 1.0.x -->
+<!-- @Mergifyio backport 2.1.x 1.0.x -->
 
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 <!-- Fixes #(issue) -->