ethereum: bump prysm v7.1.4 → v7.1.6 on mainnet only

Update Prysm beacon-chain image default for ethereum mainnet profiles
(geth pruned/minimal + reth trace); sepolia and hoodi remain on v7.1.4.

Co-authored-by: Cursor <cursoragent@cursor.com>

ethereum/reth/ethereum-mainnet-reth-archive-trace.yml

@@ -108,7 +108,7 @@ services:
     shm_size: 2gb
 
   ethereum-mainnet-reth-node:
-    image: ${ETHEREUM_PRYSM_IMAGE:-prysmaticlabs/prysm-beacon-chain}:${ETHEREUM_MAINNET_PRYSM_VERSION:-v7.1.4}
+    image: ${ETHEREUM_PRYSM_IMAGE:-prysmaticlabs/prysm-beacon-chain}:${ETHEREUM_MAINNET_PRYSM_VERSION:-v7.1.6}
     ports:
       - 19563:19563
       - 19563:19563/udp