Update dependency mini-css-extract-plugin to ^2.8.0 #2271

Job	Run time
Static analysis	1m 31s
Dependencies, Unit tests (16)	1m 32s
Dependencies, Unit tests (18)	1m 26s
Dependencies, Unit tests (20)	1m 24s
Build application (govuk-docs)	2m 28s
Build application (govuk-template)	1m 36s
Build libraries	1m 56s
Test application (govuk-docs, chromium)	1m 48s
Test application (govuk-docs, firefox)	1m 22s
Test application (govuk-docs, electron)	3m 32s
Test application (govuk-template, chromium)	1m 54s
Test application (govuk-template, firefox)	1m 30s
Test application (govuk-template, electron)	1m 36s
	23m 35s

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