Update CHANGELOG with release notes

[romaricpascal]

Replaces the content of the "Unreleased" section of the CHANGELOG with the drafted release notes (internal link).

Closes #4181

[domoscargin]

🦣

Nice stuff

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-5.0.0-internal.0.min.css	114.01 KiB
dist/govuk-frontend-5.0.0-internal.0.min.js	37.86 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	76.77 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	72.08 KiB
packages/govuk-frontend/dist/govuk/all.mjs	3.8 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.99 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	37.84 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.3 KiB

Modules

File	Size
all.mjs	68.41 KiB
components/accordion/accordion.mjs	21.05 KiB
components/button/button.mjs	4.18 KiB
components/character-count/character-count.mjs	20.49 KiB
components/checkboxes/checkboxes.mjs	5.31 KiB
components/error-summary/error-summary.mjs	5.49 KiB
components/exit-this-page/exit-this-page.mjs	15.49 KiB
components/header/header.mjs	3.39 KiB
components/notification-banner/notification-banner.mjs	4.02 KiB
components/radios/radios.mjs	4.31 KiB
components/skip-link/skip-link.mjs	3.3 KiB
components/tabs/tabs.mjs	9.07 KiB

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