doc: release notes

doc/release-notes-31600.md

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+Updated RPCs
+---
+- the `getblocktemplate` RPC `curtime` (BIP22) and `mintime` (BIP23) fields now
+  account for the timewarp fix proposed in BIP94 on all networks. This ensures
+  that, in the event a timewarp fix softfork activates on mainnet, un-upgraded
+  miners will not accidentally violate the timewarp rule. (#31376, #31600)
+
+As a reminder, it's important that any software which uses the `getblocktemplate`
+RPC takes these values into account (either `curtime` or `mintime` is fine).
+Relying only on a clock can lead to invalid blocks under some circumstances,
+especially once a timewarp fix is deployed.