Drop support for Python 3.8

[xrmx]

Python 3.8 has been EOL'ed in 2024-10-07 which is near the end of our 6 months support promise.
So we can think of dropping it and save some CI time.

We have a checklist of things to do in CONTRIBUTING when bumping our baseline.

Double check that list and update it if needed (does ruff have a baseline too?). Also maybe take a look at pyupgrade to handle any python feature we can now leverage.

Also we can bump the pypy version to something more modern like 7.3.16.

This also require changes in -contrib.

[xrmx]

From a quick grep we can cleanup these checks:

opentelemetry-sdk/tests/metrics/exponential_histogram/test_exponent_mapping.py:if version_info >= (3, 9):
opentelemetry-sdk/tests/metrics/exponential_histogram/test_exponent_mapping.py:        version_info < (3, 9),

More wins in -contrib

[jomcgi]

Hey! is this available for me to pickup?
It sounds like a good opportunity to get more familiar with the codebase.

[emdneto]

@jomcgi Code reviews are also very welcome and a good way to get used to the codebase 😅

[jomcgi]

Agreed - I'll start reviewing PRs.
Also still happy to pick up this task