Drop support for Python 3.8

[miguelgrinberg]

No description provided.

[pquentin]

git grep 3\\.8 gives other things to change:

docs/reference/getting-started.md:* [Python](https://www.python.org/) 3.8 or newer
examples/fastapi-apm/dockerfiles/Dockerfile.app:FROM python:3.8
examples/fastapi-apm/dockerfiles/Dockerfile.ping:FROM python:3.8
utils/dsl-generator.py:                    # When dropping Python 3.8, use `removesuffix("Function")` instead

Additionally, running pyupgrade --py38-plus **/*.py produces a few more interesting changes.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/getting-started.md

[pquentin]

Thanks! LGTM.

[github-actions]

The backport to 9.1 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.1 9.1
# Navigate to the new working tree
cd .worktrees/backport-9.1
# Create a new branch
git switch --create backport-3001-to-9.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 14bfbf26a38112df6ba7f058f316a8d123219b39
# Push it to GitHub
git push --set-upstream origin backport-3001-to-9.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.1

Then, create a pull request where the base branch is 9.1 and the compare/head branch is backport-3001-to-9.1.