Update header screenshots for new alpha/beta style

The alpha/beta banners were updated in alphagov/govuk_frontend_toolkit@e295f45f59495f8e37e37a105da6d9d8b2695a54,
so redo the screenshots with those new styles.

See also: https://github.com/alphagov/govuk_frontend_toolkit/pull/112
Merge pull request #393 from alphagov/update-alpha-beta-banners

Update header screenshots for new alpha/beta style