Upgrade test

[okekefrancis112]

• Added a new test legacy_closed_channel_update_id_upgrade that verifies the upgrade path for nodes carrying legacy u64::MAX update IDs from pre-0.1 LDK.

Closes: #4104

[ldk-reviews-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.04%. Comparing base (2424c4a) to head (cff6161).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4397      +/-   ##
==========================================
+ Coverage   86.03%   86.04%   +0.01%     
==========================================
  Files         156      156              
  Lines      103092   103178      +86     
  Branches   103092   103178      +86     
==========================================
+ Hits        88694    88779      +85     
- Misses      11885    11888       +3     
+ Partials     2513     2511       -2     

Flag	Coverage Δ
tests	86.04% <100.00%> (+0.01%)	⬆️

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[TheBlueMatt]

Because review bandwidth is rather limited, please don't open a flood of PRs at once - give each new PR some time to get review cycles before you open up new ones. If you're looking for ways to contribute, try reviewing existing open PRs! Learning the codebase through review is likely to be way more fruitful than just writing code.

[TheBlueMatt]

By "upgrade test", the issue meant adding a test in the lightning-tests crate which actually uses an old version of LDK to get us into a state where we have to handle u64::MAX updates.