fix: type QueryGenerator.dropForeignKeyQuery and add tests

[WikiRik]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description Of Change

I noticed in #15454 that the dropForeignKeyQuery for DB2 was not working at all. This PR types that function, fixes it for DB2 and adds tests for it.

[evanrittenhouse]

Nice find - LGTM, will approve when marked ready for review