Deprecate MTROnboardingPayloadParser.

[bzbarsky-apple]

We forgot to do this when we deprecated the QRCode/Manual parsers.

[github-actions]

PR #26272: Size comparison from 8cdd0aa to 6c861d3

Full report (1 build for cc32xx)

platform	target	config	section	8cdd0aa	6c861d3	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	601114	601114	0	0.0
			(read/write)	204132	204132	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197544	197544	0	0.0
			.comment	206	206	0	0.0
			.data	1468	1468	0	0.0
			.debug_abbrev	956756	956756	0	0.0
			.debug_aranges	103416	103416	0	0.0
			.debug_frame	349704	349704	0	0.0
			.debug_info	19489249	19489249	0	0.0
			.debug_line	2678035	2678035	0	0.0
			.debug_line_str	513	513	0	0.0
			.debug_loc	33340	33340	0	0.0
			.debug_loclists	1501882	1501882	0	0.0
			.debug_ranges	4984	4984	0	0.0
			.debug_rnglists	96008	96008	0	0.0
			.debug_str	3024877	3024877	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104170	104170	0	0.0
			.shstrtab	265	265	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	477531	477531	0	0.0
			.symtab	285936	285936	0	0.0
			.text	494824	494824	0	0.0