chore: release v0.20.0

[hugrbot]

This release contains a big list of changes reworking multiple core definitions of HUGR.
Some of these changes include:

• 🧹 Cleanup of the internal hugr traits, making it possible to create virtual
  Hugrs that are not backed by a portgraph or use different node index types.
• 🚪 The functional root of a Hugr is now called the entrypoint. This node may
  exist somewhere inside the region hierarchy, so we can now model edges that
  connect beyond the descendants of the entrypoint.
• 🌲 The hugr region hierarchy is now always rooted by a Module operation.
• ✂ Made SimpleReplacement even simpler and easy to use.
• 🔧 The Rewrite trait has been split into VerifyPatch and Patch parts.
• ✉️ Hugr and package serialization is now done entirely using Envelopes. We
  provide some backward-compatibility layers that will be removed in the future.
• 🦀 The Minimum Supported Rust Version (MSRV) has been updated to 1.85.

🤖 New release

• hugr-model: 0.19.0 -> 0.20.0 (⚠ API breaking changes)
• hugr-core: 0.15.4-> 0.20.0 (⚠ API breaking changes)
• hugr-llvm: 0.15.4 -> 0.20.0 (⚠ API breaking changes)
• hugr-passes: 0.15.4 -> 0.20.0 (⚠ API breaking changes)
• hugr: 0.15.4-> 0.20.0 (⚠ API breaking changes)
• hugr-cli: 0.15.4 -> 0.20.0 (⚠ API breaking changes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.03%. Comparing base (b8b11cf) to head (0500e5d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2218   +/-   ##
=======================================
  Coverage   82.03%   82.03%           
=======================================
  Files         230      230           
  Lines       41017    41017           
  Branches    37118    37118           
=======================================
  Hits        33648    33648           
  Misses       5399     5399           
  Partials     1970     1970           

Flag	Coverage Δ
python	85.63% <ø> (ø)
rust	81.65% <ø> (ø)

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[aborgna-q]

Open to suggestions for things to add to the high-level change list

[ss2165]