Inter UI and roboto mono font stack. Remove K6 theming. We did it - #29152

Summary

This PR does the following things:

1. Removes references to K6 EUI, using the default UI theme in its place
2. Removes open sans from Kibana. Replaces it with Inter UI and Robot Mono
3. Licenses added for fonts; readme and notes added for usage

For outside team pings

• In security some font sizing was changed. Snaps were updated as well.
• In canvas I moved the markdown file to ui/public. We have a few different markdown files, and the Canvas one is the best. It is unchanged from the canvas usage and will not cause breaks there.
• In APM snapshots were updated due to the EUI update.

Notes

I included the full stack of files that Google fonts and Inter UI provided. In many cases these files will not be used (either because a variable file is loaded, or we're not calling the font-face directly), but generally having lived in the Kibana font code for 2 years I've found it prudent to include the full set since these are versioned and you never know when you'll need them. It gives us some room should we need it later.

Testing

I tested this on Chrome, Firefox, Safari and IE11. Firefox has the best tooling for fonts if you're looking to get designy with them. I did not do a full visual check of this stuff. I think @cchaos, who is more familiar with these fonts will do a better job there. But they are rendering at the weights I expect.

Stuff to check

Because the fonts are different, anything that may have used a fixed width or height based on "eyeball" values has the potential to break. For example, if you sized a column because "that looks about 300px of content" rather than making a calculation based on variables, that column may no be 300px.

K6's base font size was 14px. K7s is 1rem/16px. In some cases you might see subtext content (like subtitles) looking large. The fix is always to just use EuiTextand apply a smaller size if that's what you think looks better. I tried to get as many instances of those in this PR as I could.

Breaking change

This is a breaking change to dashboards for 7.0. It will cause pixel level height differences vs K6 because of differences in font size and weight. We'll need to update the baseline screenshots to match.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately