Update CloudWatch custom logs ebextensions to use CloudWatch agent

[praegt]

Related issue: https://github.com/aws/elastic-beanstalk-roadmap/issues/313

Description of changes:

• Update CloudWatch custom logs ebextensions to use CloudWatch agent instead of deprecated CloudWatch Logs agent. This version should work on AL2 and AL2023 platforms.
• Update the comments to reflect the update.
• Remove package installation because amazon-cloudwatch-agent should be installed by default.
• Link to the appropriate CloudWatch documentation.
• Change "can" to "should" be used additionally to the "Log Streaming" feature. To use without enabling log streaming, I expect there might need to be modifications to this sample, such as using fetch-config instead of append-config.
• Remove /var/log/dmesg from the example because this file is not there by default on AL2023.
• Add example of configuring retention period.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[mrenneke-amazon]

@praegt - Thanks for this update! Merging.