Signal splice prototype feature bit instead

[wpaulino]

As our splice implementation is not yet complete (missing RBF support) and we still need to do cross-compatibility testing with eclair/CLN, we signal the agreed upon prototype feature bit instead.

Fixes #4062.

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[TheBlueMatt]

Trivial enough, landing.

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[TheBlueMatt]

Oops, ln::msgs::tests::encoding_commitment_signed needs updating for the new tlv

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.75%. Comparing base (fcb1164) to head (bddd5c7).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4156   +/-   ##
=======================================
  Coverage   88.74%   88.75%           
=======================================
  Files         180      180           
  Lines      135809   135821   +12     
  Branches   135809   135821   +12     
=======================================
+ Hits       120523   120546   +23     
+ Misses      12519    12506   -13     
- Partials     2767     2769    +2     

Flag	Coverage Δ
fuzzing	21.71% <ø> (+0.01%)	⬆️
tests	88.59% <100.00%> (+<0.01%)	⬆️

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