[release/v1.13.x] Add 1.13.0 release date to the change log (#5874)

* Add 1.13.0 release date to the change log

* Update CHANGELOG.md

Co-authored-by: opentelemetry-java-bot <[email protected]>
Co-authored-by: Trask Stalnaker <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## Version 1.13.0 (unreleased)
+## Version 1.13.0 (2022-04-19)
 
 ### Migration notes