Graph: Fix legend overlapping graph area

[ashharrison90]

What is this feature?

• react 18 removed the concept of render callbacks
• the quick/lazy option was to use requestIdleTimeout, but i didn't realise it wasn't supported in safari 😬
• instead, let's implement the less lazy option of passing a render callback to the legend as recommended here
• this is kind of ugly, but the graph panel is very deprecated and will be gone in grafana 11 🤷‍♂️

Why do we need this feature?

• correctly render a graph panel with legend
• stop using requestIdleTimeout which isn't supported in safari

Who is this feature for?

• anyone still using graph panel in grafana 10

Which issue(s) does this PR fix?:

Supersedes #67840

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[grafanabot]

Hello @ashharrison90!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[simPod]

I've tested this on our instance and can confirm it works as expected.