fix(button): use flexbox instead of font-size to ensure the icon is centered #4967

Job	Run time
build	3m 47s
setup-matrix	5s
lint	1m 46s
test	1m 27s
e2e (1, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_boolean.feature, ../ui/...	5m 38s
e2e (2, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_value.feature, ../ui/co...	4m 46s
e2e (3, ../ui/collections/forms/src/CheckboxFieldFF/features/displays_error.feature, ../ui/collec...	5m 6s
e2e (4, ../ui/collections/forms/src/FileInputFieldFF/features/accepts_file.feature, ../ui/collect...	4m 44s
e2e (5, ../ui/collections/forms/src/FileInputFieldFF/features/displays_error.feature, ../ui/colle...	5m 12s
e2e (6, ../ui/collections/forms/src/InputFieldFF/features/can_set_a_value.feature, ../ui/collecti...	5m 20s
e2e (7, ../ui/collections/forms/src/InputFieldFF/features/displays_error.feature, ../ui/collectio...	4m 52s
publish	1m 7s
	43m 50s

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