Explicitly pin to menhir 20231231 #13322

Job	Run time
flambda2_runtime5	13m 24s
flambda2_dev	8m 52s
flambda2_dev_runtime5	12m 16s
flambda2_debug_runtime5	19m 16s
flambda2_debug_runtime	12m 19s
flambda2_o3	10m 33s
flambda2_o3_heap	11m 7s
flambda2_o3_advanced_meet_frame_pointers_runtime5_polling	14m 14s
flambda2_o3_advanced_meet_frame_pointers_runtime5_debug	19m 10s
flambda2_frame_pointers_oclassic_polling	10m 19s
flambda2_macos	26m 24s
irc	10m 37s
irc_frame_pointers	10m 48s
ls	10m 42s
gi	10m 48s
	3h 20m 49s

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