Updates to the CHANGELOG preparing for final v3.8.0 release

Author: kmuriki

CHANGELOG.md

@@ -10,7 +10,9 @@ _With the release of `v3.0.0`, we're introducing a new changelog format in an at
 _The old changelog can be found in the `release-2.6` branch_
 
 
-# v3.8.0-rc.2 Release Candidate - [2021-05-26]
+# v3.8.0 - [2021-06-15]
+
+## Changed defaults / behaviours
 
 > :warning: Go module was renamed from `github.com/sylabs/singularity` to `github.com/hpcng/singularity`