Rename AbstractSpan to Span for consistent access to static methods

[Nevay]

Unifies access to static interface methods. Other API interfaces with static methods have a class without the Interface suffix that provides an implementation, e.g.:
Baggage::getCurrent(): BaggageInterface
SpanContext::getInvalid(): SpanContextInterface

I considered creating a Span class that does not implement SpanInterface but disliked the idea of splitting the PHPDoc documentation in different classes.

[codecov]

Codecov Report

Merging #824 (48cf613) into main (c000590) will decrease coverage by 0.01%.
The diff coverage is 36.66%.

❗ Current head 48cf613 differs from pull request most recent head 057d864. Consider uploading reports for the commit 057d864 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #824      +/-   ##
============================================
- Coverage     82.58%   82.56%   -0.02%     
  Complexity     1884     1884              
============================================
  Files           230      231       +1     
  Lines          4898     4899       +1     
============================================
  Hits           4045     4045              
- Misses          853      854       +1     

Flag	Coverage Δ
7.4	82.57% <36.66%> (-0.02%)	⬇️
8.0	82.60% <36.66%> (-0.02%)	⬇️
8.1	82.60% <36.66%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
src/API/Trace/AbstractSpan.php	0.00% <0.00%> (ø)
src/API/Trace/NonRecordingSpan.php	100.00% <ø> (ø)
src/API/Trace/Span.php	0.00% <0.00%> (ø)
src/SDK/Trace/Span.php	94.23% <ø> (ø)
src/API/Trace/NoopSpanBuilder.php	100.00% <100.00%> (ø)
...c/API/Trace/Propagation/TraceContextPropagator.php	100.00% <100.00%> (ø)
src/Extension/Propagator/B3/B3MultiPropagator.php	100.00% <100.00%> (ø)
src/Extension/Propagator/B3/B3SinglePropagator.php	98.55% <100.00%> (ø)
src/SDK/Metrics/Exemplar/BucketStorage.php	100.00% <100.00%> (ø)
.../ExemplarFilter/WithSampledTraceExemplarFilter.php	100.00% <100.00%> (ø)

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[brettmc]

LGTM. Can you please add a BC compatibility entry for this one, as it's likely to break a lot of user code.