feat(readme): migrate ReadmeCard to EntityInfoCard

[anicke]

Summary

Migrates ReadmeCard from the legacy MUI-based InfoCard (from @backstage/core-components) to EntityInfoCard (from @backstage/plugin-catalog-react), following the same pattern adopted upstream in the main Backstage repo for catalog card components.

Changes

• ReadmeCard now uses EntityInfoCard (BUI-based) by default, with the fullscreen icon button placed in headerActions.
• A new ReadmeCardLegacyProps type is introduced for backward compatibility. It retains the deprecated variant and maxHeight props and continues to render using InfoCard. Users passing either prop will automatically get the legacy rendering path.
• ReadmeCardProps is simplified to only hideIfNotFound.
• ReadmeCardLegacyProps is exported from the package's public API.

Migration

Users who relied on variant or maxHeight don't need to change anything — the legacy props are still accepted and will use the old InfoCard rendering. The new default uses EntityInfoCard.

Checklist

• Tests pass
• API report updated
• Changeset added (minor — ReadmeCardProps shape changed)

[changeset-bot]

🦋 Changeset detected

Latest commit: 66875fa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@axis-backstage/plugin-readme	Minor
app-next	Patch
app	Patch
backend	Patch

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[fridajac]

Nice!