Restore the hidden --web3-force-polling option as a deprecated one

beacon_chain/conf.nim

@@ -176,6 +176,11 @@ type
       desc: "A directory containing era files"
       name: "era-dir" .}: Option[InputDir]
 
+    web3ForcePolling* {.
+      hidden
+      desc: "Force the use of polling when determining the head block of Eth1 (obsolete)"
+      name: "web3-force-polling" .}: Option[bool]
+
     web3Urls* {.
       desc: "One or more execution layer Engine API URLs"
       name: "web3-url" .}: seq[EngineApiUrlConfigValue]

beacon_chain/nimbus_beacon_node.nim

@@ -1806,6 +1806,7 @@ proc doRunBeaconNode(config: var BeaconNodeConf, rng: ref HmacDrbgContext) {.rai
   ignoreDeprecatedOption terminalTotalDifficultyOverride
   ignoreDeprecatedOption optimistic
   ignoreDeprecatedOption validatorMonitorTotals
+  ignoreDeprecatedOption web3ForcePolling
 
   createPidFile(config.dataDir.string / "beacon_node.pid")