Disable fee estimates for a confirm target of 1 block

Backport of #9239 without GUI changes and fixing conflicts in tests.

src/rpc/mining.cpp

@@ -791,6 +791,8 @@ UniValue estimatefee(const UniValue& params, bool fHelp)
             "\n"
             "A negative value is returned if not enough transactions and blocks\n"
             "have been observed to make an estimate.\n"
+            "-1 is always returned for nblocks == 1 as it is impossible to calculate\n"
+            "a fee that is high enough to get reliably included in the next block.\n"
             "\nExample:\n"
             + HelpExampleCli("estimatefee", "6")
             );