Remove the need to transition layer height on zoom

[richardwestenra]

Description

Modifying DOM attributes is expensive. Transitioning height is also expensive. This PR removes a CSS height transition and a DOM update, because the height isn't strictly necessary for this effect. Now the label is centred, the gradient is on the container, and the translation is the only thing that gets animated, which should help performance a little.

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

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