Unity canvas containing WebGL player unable to gain focus when clicking or tabbing into it

Reproduction steps:
1. Open and run the reproduction project (to be attached in comments).
2. Build and run, targeting WebGL.
3. Try clicking on the canvas and inputting text.

Expected result: Clicking on the canvas puts it in focus, and allows input like keyboard to be captured.
Actual result: The canvas never gets focus, and all keyboard input is ignored.

Reproducible with: 2022.2.0a12
Could not test with: Did not test with older versions - should be tested.

Notes:

• The bug only occurs when WebGLInput.captureAllKeyboardInput is set to false, otherwise the global handlers capture the input.
• This seems a direct result of the canvas not having a `tabindex` in the templates. We could test if the canvas has the property defined, and attach one with the value of -1 if not.