[EuiSideNav] ID generation isn't server-side rendering safe

[chandlerprall]

Discovered while testing a version upgrade in the next-eui starter: https://github.com/elastic/next-eui-starter/pull/47#pullrequestreview-818091331 , the sideNavContentId value used for an id+aria-controls isn't server-side rendering safe.

[chandlerprall]

See https://github.com/facebook/react/issues/5867 for a definition of the problem space. Thankfully, React 18 ships with a useId hook to solve - we'll need to wait until we drop support for React@17 to fix this. However, apart from being a hard to ignore warning, this id mismatch does not cause any issues.

[github-actions[bot]]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.