Merge bitcoin/bitcoin#28920: wallet: birth time update during tx scanning

1ce45baed7 rpc: getwalletinfo, return wallet 'birthtime' (furszy)
83c66444d0 test: coverage for wallet birth time interaction with -reindex (furszy)
6f497377aa wallet: fix legacy spkm default birth time (furszy)
75fbf444c1 wallet: birth time update during tx scanning (furszy)
b4306e3c8d refactor: rename FirstKeyTimeChanged to MaybeUpdateBirthTime (furszy)

Pull request description:

  Fixing #28897.

  As the user may have imported a descriptor with a timestamp newer
  than the actual birth time of the first key (by setting 'timestamp=now'),
  the wallet needs to update the birth time when it detects a transaction
  older than the oldest descriptor timestamp.

  Testing Notes:
  Can cherry-pick the test commit on top of master. It will fail there.

ACKs for top commit:
  Sjors:
    re-utACK 1ce45baed7
  achow101:
    ACK 1ce45baed7

Tree-SHA512: 10c2382f87356ae9ea3fcb637d7edc5ed0e51e13cc2729c314c9ffb57c684b9ac3c4b757b85810c0a674020b7287c43d3be8273bcf75e2aff0cc1c037f1159f9

src/wallet/scriptpubkeyman.cpp

@@ -710,7 +710,7 @@ void LegacyScriptPubKeyMan::UpdateTimeFirstKey(int64_t nCreateTime)
         // Cannot determine birthday information, so set the wallet birthday to
         // the beginning of time.
         nTimeFirstKey = 1;
-    } else if (!nTimeFirstKey || nCreateTime < nTimeFirstKey) {
+    } else if (nTimeFirstKey == UNKNOWN_TIME || nCreateTime < nTimeFirstKey) {
         nTimeFirstKey = nCreateTime;
     }