Merge bitcoin/bitcoin#31600: rpc: have getblocktemplate mintime account for timewarp

e1676b08f7 doc: release notes (Sjors Provoost)
0082f6acc1 rpc: have mintime account for timewarp rule (Sjors Provoost)
79d45b10f1 rpc: clarify BIP94 behavior for curtime (Sjors Provoost)
0713548137 refactor: add GetMinimumTime() helper (Sjors Provoost)

Pull request description:

  #30681 fixed the `curtime` field of `getblocktemplate` to take the timewarp rule into account. However I forgot to do the same for the `mintime` field, which was hardcoded to use `pindexPrev->GetMedianTimePast()+1`.

  This PR adds a helper `GetMinimumTime()` and uses it for the `mintime` field.

  #31376 changed the `curtime` field to always account for the timewarp rule. This PR maintains that behavior.

  Note that `mintime` now always applies BIP94, including on mainnet. This makes future softfork activation safer.

  It could be backported to v28.

ACKs for top commit:
  fjahr:
    tACK e1676b08f7
  achow101:
    ACK e1676b08f7
  darosior:
    utACK e1676b08f7 on the code changes
  tdb3:
    brief code review re ACK e1676b08f7
  TheCharlatan:
    ACK e1676b08f7

Tree-SHA512: 0e322d8cc3b8ff770849bce211edcb5b6f55d04e5e0dee0657805049663d758f27423b047ee6363bd8f6c6fead13f974760f48b3321ea86f514f446e1b23231c

test/functional/mining_basic.py

@@ -153,6 +153,8 @@ class MiningTest(BitcoinTestFramework):
         # The template will have an adjusted timestamp, which we then modify
         tmpl = node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
         assert_greater_than_or_equal(tmpl['curtime'], t + MAX_FUTURE_BLOCK_TIME - MAX_TIMEWARP)
+        # mintime and curtime should match
+        assert_equal(tmpl['mintime'], tmpl['curtime'])
 
         block = CBlock()
         block.nVersion = tmpl["version"]