Deprecate then Remove `-ddump-json`

This is an issue to track the deprecation and removal of the -ddump-json flag, which is being replaced by the -fdiagnostics-as-json flag. Details of the discussion can be found at #19278.

Tasks:

Any code that uses MCDiagnostic (directly or via mkMCDiagnostic or errorDiagnostic) side steps -fdiagnostics-as-json. To address this:

1. Refactor all places in the compiler that report diagnostics to use printMessages / reportError / reportDiagnostic from GHC.Driver.Errors.
2. Get rid of errorDiagnostic
3. Rename mkMCDiagnostic to unsafeMCDiagnostic
4. Hide MCDiagnostic