Deprecate -ddump-json and introduce -fdiagnostics-as-json

Addresses #19278

This commit deprecates the underspecified -ddump-json flag and
introduces a newer, well-specified flag -fdiagnostics-as-json.

Also included is a JSON schema as part of the documentation.

The -ddump-json flag will be slated for removal shortly after this merge.

compiler/GHC/Utils/Logger.hs

@@ -463,7 +463,7 @@ defaultLogActionHPutStrDoc logflags asciiSpace h d
 -- -fdiagnostics-as-json. For temporary backwards compatibility while
 -- -ddump-json is being deprecated, `jsonLogAction` has been added in, but
 -- it should be removed along with -ddump-json. Similarly, the guard in
--- `defaultLogAction` should be removed.
+-- `defaultLogAction` should be removed. This cleanup is tracked in #24113.
 
 -- | Default action for 'dumpAction' hook
 defaultDumpAction :: DumpCache -> LogAction -> DumpAction