polywrap.app.yaml and polywrap.plugin.yaml should become polywrap.yaml

[pileks]

Is your feature request related to a problem? Please describe.
As part of the manifest refactor (#1057) we have virtually unified the Polywrap, App and Plugin manifests, with a few minor exceptions.
As part of those changes, we want to communicate that polywrap.yaml is the default filename for all project manifests (including app and plugin projects).

Describe the solution you'd like
The Polywrap CLI help files should only show polywrap.yaml | polywrap.yml | polywrap.json as the default project manifest files.
polywrap.app.* and polywrap.plugin.* should still be silently supported, but if they are detected, the user should be warned about them getting depecated in favor of polywrap.*.
All app and plugin project templates should also have their project manifest files renamed to polywrap.yaml.

Describe alternatives you've considered
A unification of the ...Manifest types has been considered (PluginManifest, AppManifest, PolywrapManifest).
This will not be considered within this issue, as the end-result of the issue should be a simple improvement to the quality of the experience a developer has when using Polywrap.

Additional context
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