refactor(developer): remove support for downlevel Keyman versions

The Keyman compiler still supports compiling to v6-v9 targets. This is really no longer necessary, as v10 was the first open source version, and earlier versions are truly EOL. We could start to deprecate support for compiling targeting older versions, which will simplify the codebase.

Downside: there are still nearly 100 keyboards around with `store(&version) '9.0'` or `VERSION 9.0` lines -- or lower. So this would need to be staged.

There are two parts to this:

1. producing down-level code, e.g. support for Caps Lock in KeymanWeb
2. supporting legacy source formats
* legacy constructs, e.g. `NAME "xyz"` vs `store(&name) 'xyz'`
* legacy keywords, e.g. `ðnologuecode`