refactor(accordion): use native CSS animations

[spliffone]

Describe in detail what your merge request does and why. Add relevant
screenshots and reference related issues via Closes #XY or Related to #XY.

---

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[gemini-code-assist]

Summary of Changes

Hello @spliffone, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the si-collapsible-panel component by migrating its expand and collapse animations from Angular's animation module to native CSS. This change aims to streamline the component's implementation by reducing TypeScript-side animation logic and delegating control to CSS properties like grid-template-rows and @starting-style, potentially leading to a more performant and maintainable animation system.

Highlights

• Animation Refactor: The accordion's expand/collapse animation has been refactored to use native CSS animations instead of Angular's @angular/animations module.
• CSS-driven Collapse: The animation now leverages CSS grid-template-rows for height transitions and @starting-style for defining the initial state of the animation, providing a more native feel.
• Simplified Component Logic: The si-collapsible-panel.component.ts file has been simplified by removing Angular animation imports and the animations metadata from the component decorator.
• Conditional Content Rendering: The collapsible content div is now conditionally rendered using an @if block, which works in conjunction with the new CSS animation approach for managing its presence in the DOM.

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[gemini-code-assist]

Code Review

This pull request refactors the collapsible panel to use native CSS animations instead of Angular's animation library, which is a great modernization. The implementation leverages display: grid and animates grid-template-rows for a smooth collapse/expand effect, which is a robust technique. The code is cleaner and has fewer dependencies. I've found one critical issue regarding the restoration of the scroll position which I've commented on. Otherwise, this is a solid improvement.

[spliffone]

@dr-itz Can you take a look? We had an issue 1183 where you adjusted the scroll behaviour but I couldn't figure out what was the problem.

[github-actions]

Documentation.
Examples.
Playwright report.

Coverage Reports:

• element-ng
• element-translate-ng

[dr-itz]

So the intention is that scrolling position is restored upon re-opening an accordion panel. In the example here: https://element.siemens.io/element-examples/#/overview/si-accordion/si-accordion-full-height?q=ac

you can make the screen small enough that the first panel will scroll. Opening another panel and then re-opening the first one restores the position. Currently broken in this MR

[github-actions]