Clock auto-selection redux

Added

• Instant.clock_name is back and now tracks clock_id using reflection.
• Explicit minimum Ruby version in gemspec (2.7.0).

Changed

• Clock auto-selection redux. We choose the first available from:

  1. CLOCK_UPTIME_RAW on macOS, faster and higher resolution, also used by Rust
  2. CLOCK_MONOTONIC
  3. CLOCK_REALTIME, a non-monotonic fallback that issues a warning on startup

• Slight performance bump for Duration.measure on Ruby <= 3.1