multi: add version to MuSig2 API, bump btcd/btcec to v2.3.2

With this commit we bump the github.com/btcd/btcec/v2 library to v2.3.2
which implements the MuSig2 BIP version v1.0.0rc2. With this the
github.com/btcsuite/btcd/btcec/v2/schnorr/musig2 package becomes
v1.0.0rc2 and the github.com/lightningnetwork/lnd/internal/musig2v040
stays at the old v0.4.0 version.

input/test_utils.go

@@ -139,7 +139,7 @@ func (m *MockSigner) ComputeInputScript(tx *wire.MsgTx, signDesc *SignDescriptor
 // all signing parties must be provided, including the public key of the local
 // signing key. If nonces of other parties are already known, they can be
 // submitted as well to reduce the number of method calls necessary later on.
-func (m *MockSigner) MuSig2CreateSession(keychain.KeyLocator,
+func (m *MockSigner) MuSig2CreateSession(MuSig2Version, keychain.KeyLocator,
 	[]*btcec.PublicKey, *MuSig2Tweaks,
 	[][musig2.PubNonceSize]byte) (*MuSig2SessionInfo, error) {