Chisel 8 Tracker

This issue exists to track work that has to be done for Chisel 8.

## Can Be Done Before Release

- [x] Deprecate `chisel3.experimental.ExtModule`, change to `chisel3.ExtModule`. (#5101)
- [x] Deprecate `chisel3.experimental.*Param`, change to `chisel3.*Param`. (#5101)
- [x] Deprecate `HasExtModule*`. (#5102)
- [x] Deprecate `BlackBox`. (#5103)
- [ ] Change deprecation on `TargetDirAnnotation` and alias in `chisel3`
- [ ] Change deprecation on `Annotation` and alias in `chisel3`
- [ ] Change deprecation on `MustDeduplicateAnnotation` and move into `chisel3`.
- [ ] Remove internal (main) uses of `os-lib` (test and build uses are fine).
- [ ] Deprecate `--use-legacy-width`
- [ ] Deprecate Double support in properties

## Right Before Release

- [ ] Move `chisel3.experimental.ExtModule` to `chisel3.ExtModule`.
- [ ] Move `chisel3.experimental.*Param` to `chisel3.*Param`.
- [ ] Remove `HasExtModule*`.
- [ ] Remove `BlackBox`.
- [ ] Remove `IntrinsicModule`.
- [ ] Delete firrtl IR
- [ ] Replace all deprecated case classes with classes
- [ ] Fix naming of some verification functions, e.g. `cover._applyWithSourceLine`
- [ ] Change Data equality to be structural for types and referential for values, get rid of BoxedData in Instantiate
- [ ] Delete deprecated ChiselStage methods
- [ ] Delete deprecated Lookupable and `@instantiable` related APIs
- [ ] Move `TargetDirAnnotation` into `chisel3`
- [ ] Move `Annotation` into `chisel3`
- [ ] Move `MustDeduplicateAnnotation` into `chisel3`.
- [ ] Delete `os-lib` as a [main] dependency (nothing against it, but it constrains users to use the same major version as us and doesn't add that much value in our own use).
- [ ] Stop publishing for Scala <= 2.13.8 (Can't publish for them with Java 17 - https://github.com/chipsalliance/chisel/pull/5202)
- [ ] Bump Mill
- [ ] Delete deprecated stuff related to Lookupable and IsInstantiable (just Modules are now instantiable?)
- [ ] Switch Chisel's logging to stderr - https://github.com/chipsalliance/chisel/pull/5213
- [ ] Delete Double support for properties
- [ ] Delete Convert stage
- [ ] Delete CIRCTConverter