Disallow absolute deadlines for relative cancel scopes

[A5rocks]

This just converts our deprecations into RuntimeErrors. It's been long enough!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (3dd35d7) to head (ef54ef3).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3403   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19418    -6     
  Branches         1318         1318           
===============================================
- Hits            19424        19418    -6     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)
src/trio/_tests/test_timeouts.py	100.00000% <100.00000%> (ø)

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[CoolCat467]

Maybe have a small snippet about how to port old code if this wasn't already mentioned in 0.27.0's release notes, though I wouldn't expect many people to encounter that since it's been depreciated for quite a bit now.