Prepare release 1.6.0

[tigarmo]

What needs to get done

Do the work to cut the next "feature" release, 1.6.0. This includes:

• Merge whatever is in-flight/review and can sensibly be merged
• Releasing new minor versions of craft-cli and craft-application
• Preparing a good user-oriented changelog describing the relevant changes in Rockcraft and and the libraries since 1.5.3
• Announce the 1.6.0 candidate and do a call for testing

Why it needs to get done

Our last release was back in July and a lot has changed in the craft libraries in the meantime.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3527.

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