(0.5) Deprecate legacy pending HTLC ChannelManager persistence

[valentinewallace]

In LDK 0.3 we're adding support for reconstructing the ChannelManager's pending
forward HTLCs maps from channel data as part of removing the requirement to
regularly persist the manager, but til 0.5 we still would write/read those maps
within the manager to maintain compat with 0.2-. Also 0.3 adds a new field to
Channel that allowed the map reconstruction.

Once a few versions have passed, we have more confidence that the new field
is being written, so here we deprecate persistence of the old legacy maps and only
attempt to read them if the manager serialized version indicates that they were
written. We can also deprecate some other old codepaths, see commit messages.

Partially addresses #4286

Based on #4303

• Test upgrading 0.2 -> 0.3/0.4 -> 0.5
• Test downgrading 0.5 -> 0.3/4 -> 0.2
• Update upgrade_downgrade_tests that currently upgrade from 0.2- to main, to upgrade to 0.3/0.4 instead (since 0.2- to 0.5 is unsupported)
• Clean up clauded'd upgrade/downgrade tests
• Update upgrade_downgrade_tests to not point to my branch once 0.3 is released

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[codecov]

Codecov Report

❌ Patch coverage is 80.52632% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.13%. Comparing base (3497b59) to head (492f5db).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	74.73%	15 Missing and 9 partials ⚠️
lightning/src/ln/channelmanager.rs	86.17%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4359      +/-   ##
==========================================
+ Coverage   86.09%   86.13%   +0.03%     
==========================================
  Files         156      156              
  Lines      103623   103456     -167     
  Branches   103623   103456     -167     
==========================================
- Hits        89211    89108     -103     
+ Misses      11895    11854      -41     
+ Partials     2517     2494      -23     

Flag	Coverage Δ
tests	86.13% <80.52%> (+0.03%)	⬆️

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