Sort out `DiscardFunding` event with tests for V2 channels

[dunxen]

Doesn't have to be in this PR, but we definitely need to get our `DiscardFunding` event story together (with tests) before this ships.

Originally posted by @TheBlueMatt in https://github.com/lightningdevkit/rust-lightning/pull/3137#discussion_r1811587272

[TheBlueMatt]

We're gonna walk back our v2 enablement for 0.1, so this needs to happen for 0.2.

[jkczyz]

FYI, dual funding will slip to 0.3 as development has been focused on splicing.

[wpaulino]

This may already be covered by #4030, but still needs tests of course.