chore: update stimulus to 0.21.4

[szhorvat]

This updates Stimulus to the latest version, fixing issues with generating empty argument lists. See igraph/stimulus#6

Once this version is used to re-generate rinterface.c, issues like this will be fixed:

rigraph/src/rinterface.c

Lines 11506 to 11507 in 9cc9560

	// FIXME: Change Stimulus to generate (void) instead of ()
	SEXP R_igraph_version(void) {

Please do merge this now but do not re-generate the code until #1207 is merged.

@krlmlr OK to add Stimulus updates to the merge queue myself in the future?

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[krlmlr]

Thanks! The mainline no longer suffers from this problem after you fixed the drift, merging anyway.