doc: fill in missing V8 version
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
The CHANGELOG file contains a placeholder "5.0.71.xx" in reference to release 6.3.1's V8 version. Fill in the correct V8 version for that release, which is "5.0.71.57."