Switch 0FC to prod feature bit

[TheBlueMatt]

@t-bast did interop here and we have test vectors. Should we go ahead and do this now, or leave it on the +100 and wait? I kinda feel like we should just do it. Tagging 0.2 so we decide this week.

Also cc @carlaKC and @tankyleo

[t-bast]

I'd like to just go ahead and use the final feature bit, but there's a risk that when cln and lnd actually take a look at it they'll request changes which will be annoying to handle. I think it's safer to use +100 for now, tell cln and lnd that we're starting to roll this out and we'd like them to take a deeper look at the spec PR because we'll start using the final feature bit soon (in a few months?).

[TheBlueMatt]

Okay. This probably won't get a lot of use immediately anyway, due to missing miner/electrum support. Will tag 0.3 so we switch by then, at which point hopefully ldk-node can start using this.