Document Airflow 3 UI browser requirements

[SeongUkKim]

Why

Older Chromium-based Edge versions can render the Airflow 3 UI incorrectly when they do not support modern CSS color functions. This was reported in #68312, and the UI documentation did not state the browser expectation.

What

• Adds a note to the UI Overview recommending current versions of Chrome, Edge, Firefox, or Safari.
• Calls out unsupported modern CSS color functions such as oklch() and color-mix() as a possible reason for incorrect UI rendering in older browsers.

Testing

• git diff --check

Related to #68312

Disclosure: This PR was prepared with Codex assistance and manually reviewed before submission.

[boring-cyborg]

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