fix: use flexbox instead of font-size to ensure the icon is centered #4959

Job	Run time
setup-matrix	5s
build	4m 14s
test	1m 34s
lint	1m 43s
e2e (1, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_boolean.feature, ../ui/...	5m 34s
e2e (2, ../ui/collections/forms/src/CheckboxFieldFF/features/can_toggle_a_value.feature, ../ui/co...	4m 56s
e2e (3, ../ui/collections/forms/src/CheckboxFieldFF/features/displays_error.feature, ../ui/collec...	4m 57s
e2e (4, ../ui/collections/forms/src/FileInputFieldFF/features/accepts_file.feature, ../ui/collect...	4m 45s
e2e (5, ../ui/collections/forms/src/FileInputFieldFF/features/displays_error.feature, ../ui/colle...	5m 14s
e2e (6, ../ui/collections/forms/src/InputFieldFF/features/can_set_a_value.feature, ../ui/collecti...	5m 21s
e2e (7, ../ui/collections/forms/src/InputFieldFF/features/displays_error.feature, ../ui/collectio...	5m 10s
publish	1m 49s
	45m 22s

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