Rename approximator.summaries to summarize with deprecation

[vpratz]

As discussed in #510.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/approximators/continuous_approximator.py	60.00%	2 Missing ⚠️
...low/approximators/model_comparison_approximator.py	60.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
...flow/diagnostics/metrics/model_misspecification.py	100.00% <100.00%> (ø)
bayesflow/approximators/continuous_approximator.py	91.66% <60.00%> (-0.89%)	⬇️
...low/approximators/model_comparison_approximator.py	84.21% <60.00%> (-2.62%)	⬇️

... and 15 files with indirect coverage changes