Atmos 1.15.0: Bump atmos to 1.15.0. Update atmos.yaml. Update example and tests

[aknysh]

what

• Bump atmos to 1.15.0
• Update atmos.yaml
• Update example and tests

why

• Update atmos "deps" calculation. In some cases, when more than one different YAML stack config files imported the same import (b/c some components in both derived from the same base component), then all of those stack config files were added to the component's "deps" list (resulting in Spacelift stacks having unrelated dependency labels which would cause unnecessary stack triggering)
• Keep up to date
• Keep the examples and tests the same as in atmos repo

references

• https://github.com/cloudposse/atmos/releases/tag/v1.15.0