Remove version field from package.json

[MatteoPologruto]

The version field is only needed if the package is to be published. In this project, package.json is used only for management of the dependencies and scripts for internal development.

For this reason, having a version field in the file is harmful because it increases the effort and complexity of making a release, and also is likely to get out of sync with the true version.

Reference:

https://docs.npmjs.com/cli/v8/configuring-npm/package-json#version

If you don't plan to publish your package, the name and version fields are optional.

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This was done last year for the arduino/setup-task action without any ill effects: arduino/setup-task#49.
The same was done for the arduino/arduino-lint-action action: arduino/arduino-lint-action#69