Fix floating labels rendering on Firefox

[julien-deramond]

Fixes #41342

This PR addresses the floating label rendering issue in Firefox, based on the discussion and analysis at #41342 (comment).

We considered three possible solutions:

1. Pin autoprefixer to the previous release. However, it's unclear how the issue will be handled upstream.
2. Add not kaios <= 2.5 to our .browserlistrc which appears to resolve the problem in Firefox. This is the solution implemented in this PR. Note: if autoprefixer provides an upstream fix, we’ll likely need to revisit and remove this workaround.
3. Identify a more robust fix by adjusting our floating label implementation or refining our use of pseudo-elements.

We chose the second option as a temporary, pragmatic fix, for a quick v5.3.5. A more permanent solution may involve revisiting our use of pseudo-elements once we better understand the root cause or once an upstream fix is released.

Live preview available at https://deploy-preview-41347--twbs-bootstrap.netlify.app/docs/5.3/forms/floating-labels/

Rendering:

• ✅ Firefox 137.0 - macOS 15.3.2
• ✅ Chrome 134.0.6998.166 (Official Build) (arm64) - macOS 15.3.2
• ✅ Safari 18.3.1 (20620.2.4.11.6) - macOS 15.3.2
• ✅ Edge 134.0.3124.95 (Official build) (arm64) - macOS 15.3.2
• ✅ Firefox 117.0 - Ubuntu 22.04