[eprh] Prepare for 7.0.0 (#34757)

For 7.0.0:

Slim down presets to just 2 configurations:

- `recommended`: legacy and flat config with all recommended rules, and
- `recommended-latest`: legacy and flat config with all recommended
rules plus new bleeding edge experimental compiler rules

Removed:
- `recommended-latest-legacy`
- `flat/recommended`

Please see the README for new install instructions.

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* #34783
* #34782
* __->__ #34757