refactor(stream): deprecate ReceiverExecutor in favor of MergeExecutor

[xxchan]

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What's changed and what's your intention?

follow up of #5311

ReceiverExecutor is a specialized MergeExecutor with a singleton input. The specialization seems not very useful, so remove it to KISS.

Benchmark:

https://rw-buildkite-stats.streamlit.app/Benchmark_My_PR%F0%9F%9A%80?pr_number=19160

http://metabase.risingwave-cloud.xyz/question/2219-nexmark-rw-compare?risingwave_tag_1=git-28c1465e6b52fb57d0ab330d4aa889ef27eab3f0&rw_label_1=benchmark_my_pr&risingwave_metrics=avg-source-output-rows-per-second&risingwave_tag_2=nightly-20241028&rw_label_2=daily

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