Remove deprecated legacy -c generate-*/log-* spellings in a future release

Follow-up to #784.

#784 restructures the generator CLI so generation features live under `--generate [=false]`, diagnostics under `--log [=false]`, and `-c`/`--config` covers only the mode families (`codegen`/`file`/`types`/`vtbls`) plus `help`. For back-compat it keeps the legacy `-c generate-*` / `-c log-*` spellings routing to their new homes, emitting a single deprecation warning per run.

Those legacy spellings are deliberately grouped/categorized as "legacy/deprecated" so they can be removed cleanly in a future release. This tracks that removal:

- Delete the `s_deprecatedConfigSwitches` routing and the deprecation-warning path in `Program.cs`.
- Remove the "Legacy/Deprecated -c spellings" section from `--config help` and the README `--config help` dump.
- `-c`/`--config` then only accepts the four mode families + `help`; an unrecognized `-c generate-*`/`log-*` becomes a hard error pointing at `--generate`/`--log`.

Should be scheduled for the next release where a breaking CLI change is acceptable.

Start in Program.cs with the s_deprecatedConfigSwitches routing and deprecation-warning path, then inspect the README --config help dump and the corresponding --config help output. Remove the legacy/deprecated entries and confirm that only the four mode families and help remain, with old generate-* and log-* spellings producing errors that point to --generate or --log.

Written by the indexing model from the issue text.