Replace AZP container image with 6.0.0

[saito-hideki]

SUMMARY

Currently, we are using "main" image, but It would be better to follow the settings of other collections.

• https://github.com/ansible-collections/news-for-maintainers/issues/71
• https://github.com/ansible-collections/ansible.posix/blob/main/.azure-pipelines/azure-pipelines.yml#L40

ISSUE TYPE

• CI tests

COMPONENT NAME

• ansible.posix

ANSIBLE VERSION

• all branches

COLLECTION VERSION

• all versions