Update dependency html-loader to ^4.2.0 #2146

Job	Run time
Static analysis	1m 49s
Dependencies, Unit tests (16)	1m 30s
Dependencies, Unit tests (18)	1m 25s
Dependencies, Unit tests (20)	1m 17s
Build application (govuk-docs)	2m 24s
Build application (govuk-template)	1m 55s
Build libraries	2m 5s
Test application (govuk-docs, chromium)	1m 34s
Test application (govuk-docs, firefox)	1m 31s
Test application (govuk-docs, electron)	1m 2s
Test application (govuk-template, chromium)	1m 35s
Test application (govuk-template, firefox)	1m 47s
Test application (govuk-template, electron)	2m 5s
	21m 59s

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