Don't merge open PRs in dev by default.

[ideadude]

We're not sure why we would want to merge open PRs in the dev branch AFTER pushing a release. This commit disables that by setting the default for the 'merge' option to ''.

This didn't seem to be an issue in the past because failing tests and branch protection rules would stop any PRs from merging anyway.

Checklist:

• [ ] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	77.18%	328/425
🟡	Branches	68.45%	141/206
🟢	Functions	82.93%	68/82
🟡	Lines	76.68%	319/416

Test suite run success

169 tests passing in 22 suites.

Report generated by 🧪jest coverage report action from ef21d6c4be2b034a85be68569b64b75a3c6437c3