drop php 8.0 support

[brettmc]

these are mostly automated changes from rector, applying 8.1 syntax updates.
also, apply some manual type-hint updates and readonly properties that rector missed.

after this is merged, we can separate create PRs to:

• upgrade phpunit to v10
• replace some interfaces/classes with enums

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 84.58%. Comparing base (01c46a2) to head (b790767).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1256      +/-   ##
============================================
- Coverage     84.61%   84.58%   -0.03%     
+ Complexity     2137     2136       -1     
============================================
  Files           284      284              
  Lines          6056     6054       -2     
============================================
- Hits           5124     5121       -3     
- Misses          932      933       +1     

Flag	Coverage Δ
8.0	?
8.1	84.58% <80.00%> (-0.01%)	⬇️
8.2	84.58% <80.00%> (-0.01%)	⬇️
8.3	84.58% <80.00%> (-0.01%)	⬇️
8.4	84.58% <80.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/API/Baggage/Baggage.php	82.14% <100.00%> (ø)
src/API/Baggage/Entry.php	0.00% <ø> (ø)
src/API/Baggage/Propagation/BaggagePropagator.php	96.87% <100.00%> (ø)
src/API/Baggage/Propagation/Parser.php	100.00% <ø> (ø)
src/API/Globals.php	93.54% <ø> (ø)
src/API/Instrumentation/CachedInstrumentation.php	100.00% <ø> (ø)
src/API/Logs/EventLogger.php	100.00% <ø> (ø)
src/API/Trace/NonRecordingSpan.php	100.00% <100.00%> (ø)
src/API/Trace/NoopSpanBuilder.php	100.00% <100.00%> (ø)
src/API/Trace/SpanContext.php	92.85% <ø> (ø)
... and 126 more

... and 1 file with indirect coverage changes

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[agoallikmaa]

Should be able to drop symfony/polyfill-php81 dependency.