clarify deadline in summary

policy/tpa-rfc-10-jenkins-retirement.md

@@ -8,7 +8,7 @@ builds operational. Non-critical websites (e.g. documentation) will be
 built by GitLab CI and served by GitLab pages. Critical websites
 (e.g. main website) will be built by GitLab CI and served by the
 static mirror system. Teams are responsible for migrating their jobs,
-with assistance from TPA.
+with assistance from TPA, by the end of the summer (September 1st 2021).
 
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