Add support for `Testnet4`

[tnull]

We simply add support for bitcoin::Network::Testnet4, which was added with release v0.32.4, which we hence deem the new minimal requirement for lightning and lightning-invoice.

[ldk-reviews-bot]

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Codecov Report

:x: Patch coverage is 0% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.33%. Comparing base (6d4897c) to head (743f43f). :warning: Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-invoice/src/lib.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #4148    +/-   ##
========================================
  Coverage   89.32%   89.33%            
========================================
  Files         180      180            
  Lines      138329   138480   +151     
  Branches   138329   138480   +151     
========================================
+ Hits       123566   123710   +144     
+ Misses      12157    12148     -9     
- Partials     2606     2622    +16     

Flag	Coverage Δ
fuzzing	35.96% <0.00%> (+0.07%)	:arrow_up:
tests	88.70% <0.00%> (+<0.01%)	:arrow_up:

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[tnull]

Since the other Lightning implementations seem to just map Testnet4 to Testnet, I now updated this PR to follow suit and not introduce a new Currency variant. Note this could lead to verification issues down the line, since we now might return a wrong Network from Bolt11Invoice::network and use a wrong network in fallback_addresses.

@TheBlueMatt @benthecarman Any opinions how to do better if the community doesn't seem to be enthused about introducing a separate prefix for testnet4?

[TheBlueMatt]

It seemed like people on the call were just viewing testnet4 LN as DoA, so not much reason to worry about supporting it well IMO.

[tnull]

Hmm, yea, this is fine. Annoying, but fine. Maybe the docs for Currency::Testnet should be updated to reference that its both?

Amended with some more docs:

> git diff-tree -U2 c49c982bb 743f43fcf
diff --git a/lightning-invoice/src/lib.rs b/lightning-invoice/src/lib.rs
index 2bc6e53d9..0dbb3bdb0 100644
--- a/lightning-invoice/src/lib.rs
+++ b/lightning-invoice/src/lib.rs
@@ -439,4 +439,6 @@ pub enum Currency {

        /// Bitcoin testnet
+       ///
+       /// Note this will also be used for invoices on [`Network::Testnet4`].
        BitcoinTestnet,

@@ -1623,4 +1625,7 @@ impl Bolt11Invoice {
        /// Returns the network for which the invoice was issued
        ///
+       /// **Caution**: On Testnet4, this method will return [`Network::Testnet`]` rather than
+       /// [`Network::Testnet4`].
+       ///
        /// This is not exported to bindings users, see [`Self::currency`] instead.
        pub fn network(&self) -> Network {

[TheBlueMatt]

CI failure looks like a flake.