Rename file_format to avoid confusion

[marcalff]

The "YAML file_format", as in this node in config.yaml:

# The file format version
file_format: "0.1"

collides with the "YAML file format" from the spec.

This introduces unnecessary confusion.

Suggested change, rename to configuration_version.

[marcalff]

Just found out other opentelemetry yaml files uses the file_format node, for example in build tools:

https://github.com/open-telemetry/build-tools/blob/main/schemas/testdata/1.13.0-example

Better to keep the same pattern everywhere then.

Closing.