[EuiIcon] Deprecate the `onIconLoad` prop

[tkajtoch]

Summary

As part of the EuiIcon refactor, we want to deprecate the onIconLoad prop. It's unused internally and serves little to no value.

Acceptance criteria

• onIconLoad prop should be marked as deprecated
• In the current class component implementation, the logic behind calling onIconLoad should stay the same; in the new function component implementation, it should be called on first render for compatibility purposes.