BREAKING: Update react-helmet-async to v2 #2625

Job	Run time
Dependencies, Unit tests (16)	1m 21s
Static analysis	1m 46s
Dependencies, Unit tests (18)	1m 18s
Dependencies, Unit tests (20)	1m 15s
Build libraries	2m 0s
Build application (govuk-docs)	2m 15s
Build application (govuk-template)	1m 30s
Test application (govuk-docs, chromium)	1m 51s
Test application (govuk-docs, firefox)	1m 16s
Test application (govuk-docs, electron)	1m 4s
Test application (govuk-template, chromium)	1m 43s
Test application (govuk-template, firefox)	1m 42s
Test application (govuk-template, electron)	1m 11s
	20m 12s

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