Keybindings: Change 'h' to 'mod+h' to open help modal

[tskarhed]

What is this feature?
Updates the help modal to open on mod+h instead of just h.

Some other cleanup updates:

• Use CSS object for styles
• Use theme.shape.radius.default for displaying keys instead of borderRadius(3)

Why do we need this feature?

WCAG recommendation is that a modifier is needed, as having just a single character may interfere with assistive technologies.

Who is this feature for?
Mainly keyboard users

Fixes #74841

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.

[JoaoSilvaGrafana]

Lgtm! 👏 But if you wanted more work to replace the css with components also feel free 😅

[frebib]

This seems to be hijacking Ctrl+H in Firefox on Linux with seemingly no way to prevent it? Can a different key combination be used, or a way provided to turn these off?

[tskarhed]

@frebib This has been fixed in #85449