Release 3.7.0 (#271)

* Update "Tested up to:" field in readme.txt

* Update "Requires at least" field to 6.1

* Update changelog of the version 3.7.0

* Revert "Update "Requires at least" field to 6.1"

This reverts commit a347604.

* Restore package-lock.json generated by NodeJS 14 + npm 6.14

Author: fjorgemota