mainnet/upgrade.json: revert quasarTimestamp → etnaTimestamp

Commit af356f7 (Etna → Quasar Edition) renamed the field in source
but the deployed EVM plugin at s3://lux-plugins/v1.1.0/ is built
from luxfi/evm@v0.18.18 which still reads NetworkUpgrades.EtnaTimestamp
(json tag etnaTimestamp). Until a new plugin ships with the renamed
struct, mainnet upgrade.json must use the pre-rename key — otherwise
the override silently drops and the plugin's verifyNetworkUpgrades
trips "etnaTimestamp not enabled, but fortunaTimestamp enabled" for
every brand L1 EVM that inherits this override structure.

Matches the on-disk content active on lux-mainnet luxd pods
(/data/configs/chains/C/upgrade.json, applied via the luxd-startup
ConfigMap patch on 2026-06-03). When the plugin upgrade lands the
key flips back to quasarTimestamp.

configs/mainnet/upgrade.json

@@ -2,7 +2,7 @@
   "networkUpgradeOverrides": {
     "evmTimestamp": 0,
     "durangoTimestamp": 0,
-    "quasarTimestamp": 0,
+    "etnaTimestamp": 0,
     "fortunaTimestamp": 0,
     "graniteTimestamp": 0,
     "strictPQTimestamp": 1766708400