Revert "lightningd: always require "jsonrpc": "2.0" in request."

[rustyrussell]

This reverts commit 43b037a.

@NicholasDorier says BTC Payserver still wants this for another year or so.

Changelog-Added: JSON-RPC: reverts requirement for "jsonrpc" "2.0" inside requests (still deprecated though, just for a while longer!)
Fixes: #5782

[NicolasDorier]

I think we are fine without it.
I tried to add jsonrpc 2.0 to our library, then tested against old node. Old node accepted it fine. I was worried to be in a situation where our library wouldn't be able to support old nodes + new nodes because of this.

I believe allowing it to be bypassed with allow-deprecated-api is the right call though, even if it doesn't seem we need it.

There are several projects in our deployment stack such as Charge, Spark and RTL that may need it, but I didn't had time to check.

[rustyrussell]

Agreed, I say merge this, it's simple (it's @cdecker decision, as Release Captain), and we can revisit once everyone has reasonably upgraded BTCPayServer.

[cdecker]

ACK 7255cd0

[NicolasDorier]

@rustyrussell I am suprised allow-deprecated-apis default is true.
I didn't see this change before because my tests wasn't setting this.
In bitcoin core, this is the reverse, and I believe this make sense: as integrator,

1. you bump,
2. you test,
3. you see it blows up,
4. set the flag for now
5. migrate to the new thing later

[cdecker]

It is defaulting to true because most users will not want their nodes to just die when a deprecated feature is being used. This is in contrast to app devs which should always run with it set to false, so they can be notified early about any issues.

If we were to set it to false by default we'd get the worst experience for users, and no heads up for app devs.