Remove legacy trampoline feature bit

[t-bast]

Since this creates issues with lnd v0.15, we fully remove this legacy feature bit (but still support reading it in invoices).

However, since our serialisation uses Kotlin serialisation for features (instead of using the spec serialisation), this breaks old serialised data that contains this feature bit. So we should only merge this PR once we're confident that all users aren't activating the legacy TrampolinePayment feature bit anymore.

The change in the compatibility tests shows that this is breaking backwards-compatibility for those users.

To minimize compatibility issues, we will integrate that change when we introduce the final, spec-compliant trampoline feature bit.