Relax `aria-valuenow` requirement on `slider` role

[adampage]

Closes #2260

Relaxes a requirement for the slider role by permitting authors to omit aria-valuenow in favor of aria-valuetext alone.

I purposefully left the adjacent “browsers MAY implement the repair techniques...” statement unchanged, as well as the slider-specific repair technique that it cross-references. Both still seemed compatible with this change.

My introduction of “or” in the slider’s Required States and Properties cell on line 7738 looks unprecedented in the spec in terms of its format. I’d be grateful for the editors to weigh in on that?

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [x] "user agent MUST" tests: N/A
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR:

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