feat(config): validate declarative config file_format version (#5315)

* feat(config): validate declarative config file_format version

Per the configuration spec's versioning rules, the loader now rejects a
file_format whose major version it does not support and warns (but accepts)
when the minor version is newer than the schema this SDK targets. Pre-release
meta tags (e.g. 1.0-rc.2) are tolerated. Previously any string was accepted.

* Add changelog fragment for #5315

* Apply suggestion from @xrmx

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>