chore(docs): rm mentions of staging testnet (#3828) #14339

Job	Run time
gno2go (1.23.x)	52s
test (1.23.x)	1m 7s
lint (1.23.x)	46s
mod-tidy (1.23.x)	2m 46s
Run gno fmt on examples / fmt	39s
Check generated files are up to date / generated	34s
	6m 44s

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