Replace `impl fn` with `override fn`

[josh11b]

This was decided in #5253, but needs implementation:

• Proposal (can be small, mostly conveying the justification from Should we change impl fn to override fn? #5253)
• Update the design docs, particularly https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/classes.md
• Update the toolchain (this in particular would be a good PR for new contributors)