docs(DEVELOPER.md): fix links to busted and luacheck

[jenstroeger]

Summary

Fix links to busted and luacheck in DEVELOPER.md

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

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[flrgh]

Looks good, thank you @jenstroeger for the fix!

Since the links are used more than once each, I this would arguably be a good use case for reference-style markdown links, but I think it's perfectly fine to merge in its current state.

[locao]

busted URL change suggested, to keep our docs consistent.

fixed

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/8956