fix: app config rename

[rajarju]

Description

Rename firefly app configuration key for .aio file to app

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #331 (4713e0d) into master (f7e43b2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #331   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           29        29           
  Lines         1270      1274    +4     
  Branches       209       210    +1     
=========================================
+ Hits          1270      1274    +4     

Impacted Files	Coverage Δ
src/lib/config-loader.js	100.00% <100.00%> (ø)

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[shazron]

Unfortunately, we will also need to be backwards compatible and support the cna property still, and print out a deprecation message it if someone accesses it. We can remove it if we publish a major version.

[rajarju]

@shazron Just what we were discussing
I have added a sample text for the warning