Enable -Wconversion in mbed platform code.

[bzbarsky-apple]

No description provided.

[github-actions]

PR #25409: Size comparison from e434c6a to 4a12a6a

Decreases (1 build for cc32xx)

platform	target	config	section	e434c6a	4a12a6a	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	2026292	20262923	-1	-0.0

Full report (3 builds for cc32xx, qpg)

platform	target	config	section	e434c6a	4a12a6a	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	643465	643465	0	0.0
			(read/write)	203688	203688	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197088	197088	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	930213	930213	0	0.0
			.debug_aranges	87336	87336	0	0.0
			.debug_frame	300024	300024	0	0.0
			.debug_info	2026292	20262923	-1	-0.0
			.debug_line	2657815	2657815	0	0.0
			.debug_loc	2800026	2800026	0	0.0
			.debug_ranges	282240	282240	0	0.0
			.debug_str	3023883	3023883	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105929	105929	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	378514	378514	0	0.0
			.symtab	256560	256560	0	0.0
			.text	535412	535412	0	0.0
qpg	lighting-app	qpg6105+debug	(read/write)	1151676	1151676	0	0.0
			.bss	99804	99804	0	0.0
			.data	852	852	0	0.0
			.text	598772	598772	0	0.0
	lock-app	qpg6105+debug	(read/write)	1118892	1118892	0	0.0
			.bss	96292	96292	0	0.0
			.data	864	864	0	0.0
			.text	565992	565992	0	0.0