server: remove accountability signal experiment period

In the previous iteration with endorsement
signaling, the recommendation was for the sender to
set it to 1 and that could have had privacy concerns
when first deploying given that the default was to
downgrade the signal to 0. In the latest proposal
the recommended default for both sending and
forwarding nodes is to set `accountable` to 0.
As a result, the dates have been removed given
that there are no privacy risks associated
with relaying the signal with zero values.

docs/release-notes/release-notes-0.21.0.md

@@ -75,6 +75,10 @@
   This applies to both funders and fundees, with the ability to override the
   value during channel opening or acceptance.
 
+* Rename [experimental endorsement signal](https://github.com/lightning/blips/blob/a833e7b49f224e1240b5d669e78fa950160f5a06/blip-0004.md)
+  to [accountable](https://github.com/lightningnetwork/lnd/pull/10367) to match
+  the latest [proposal](https://github.com/lightning/blips/pull/67).
+
 ## RPC Updates
 
 ## lncli Updates