refactor: make `boto3` an optional dependency under `s3` extra

[stabldev]

changes:

• [X] removed boto3 from main dependencies to reduce base install footprint for non-S3 users
• [X] added [project.optional-dependencies.s3] section with updated boto3 version
• [X] changed test import from moto.mock_s3 (deprecated) to moto.mock_aws
• [ ] update readme and documentation