Switch to the newer key for the Local SEO version

[enricobattocchi]

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where the DB version for Local SEO would not be updated correctly.

Relevant technical choices:

• the key in wpseo_local storing the Local SEO version has changed from version to local_version since 12.3, but Test Helper was never adapted to reflect that.

Milestone

• I've attached the next release's milestone to this pull request.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Install and activate Local SEO
• Use Test Helper to change the DB version for Local SEO to a previous version e.g. 14.9
• See that on reload the version is back to the actual one of the installed Local SEO (e.g. 15.0) because Local SEO has adjusted it.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Fixes #