ci: stop Dependabot bumping boto3/botocore past aioboto3's cap

[ServerSideHannes]

Root cause of the "Dependabot can't resolve your Python dependency files" error

The Dependabot resolver log:

× No solution found ...
  Because aiobotocore==2.25.1 depends on botocore>=1.40.46,<1.40.62 and
  aioboto3==15.5.0 depends on aiobotocore[boto3]==2.25.1 ...
  And because your project depends on botocore==1.43.17 ...
  we can conclude that your project's requirements are unsatisfiable.

• aioboto3 15.5.0 is the latest release and hard-pins aiobotocore==2.25.1, which caps botocore <1.40.62.
• Even the newest aiobotocore (3.7.0) only allows botocore <1.43.1, so botocore 1.43.17 is too new for the entire aioboto3 stack.
• The lockfile holds botocore at 1.40.61 today, which works — but Dependabot's group kept trying to push boto3/botocore to latest, which can't resolve.

Fix

ignore boto3 and botocore so Dependabot doesn't bump them independently — aioboto3 governs their versions (uv still resolves a compatible botocore in the lockfile). Everything else continues to update normally.

Remove the ignore once a newer aioboto3 ships that supports current botocore.