Bump postcss, css-loader and optimize-css-assets-webpack-plugin in /apps/event_stream/assets #3180

Job	Run time
Build and Cache deps	1m 8s
Credo	23s
ESLint	49s
Dialyzer static analysis	54s
Code formatting checks	19s
Missing translation keys check	39s
JS Tests	43s
Sobelow security analysis	29s
Event Stream Tests	34s
Blockscout Web Tests	3m 56s
Explorer Tests	2m 36s
EthereumJSONRPC Tests	1m 17s
Indexer Tests	1m 58s
Publish Unit Tests Results	15s
set-docker-vars	0s
Build-Blockscout-api	0s
Build-Blockscout	0s
	16m 0s

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