[0.2] Move V2 channel opens behind a cfg flag again

[TheBlueMatt]

While its somewhat sad to do, we didn't really make the progress we wanted on dual funded channels in 0.2, and thus aren't super confident in them. Further, because we still don't support actually contributing inputs to an inbound channel nor outbound v2 channel opens there's not really any reason to support them.

Thus, we simply re-cfg-gate them here for 0.2.

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[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.80%. Comparing base (21e9a9c) to head (ac8aa68).

Files with missing lines	Patch %	Lines
lightning/src/util/config.rs	53.84%	0 Missing and 12 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              0.2    #4157   +/-   ##
=======================================
  Coverage   88.79%   88.80%           
=======================================
  Files         180      180           
  Lines      136553   136566   +13     
  Branches   136553   136566   +13     
=======================================
+ Hits       121259   121277   +18     
  Misses      12494    12494           
+ Partials     2800     2795    -5     

Flag	Coverage Δ
fuzzing	21.63% <53.84%> (+0.02%)	⬆️
tests	88.64% <100.00%> (+<0.01%)	⬆️

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[TheBlueMatt]

Grrr, my attempt at cfg-flagging the config read so that the fuzzer doesn't change didn't work cause rustc invented a type out of thin air rather than failing...I set it to u8 so it works now.