Update platform_release format

[hamza-itential]

Updates the format of the platform_release version to require the Platform major version only (i.e. 6 rather than 6.0 or 6.1).

This involved renaming files (platform_release_6.0.yaml to platform_release_6.yaml, 6.0-properties.j2 to 6-properties.j2) and updating the README and other documentation accordingly.

[kvelarde-itential]

Steven and I discussed adding an extra line to the ansible.builtin.include_vars tasks. Let's discuss the proposed change and and see if it makes sense.

[hamza-itential]

Added logic to look for files matching the entire version including minor release, and then fall back to the major release only if not found. This handles all edge cases I can think of:

• Passing a supported major release only: The file is immediately found by the "platform-release-{{ platform_release }}.yml" query.
• Passing an unsupported major release only: The first query obviously fails; the second fails gracefully too since spltting a string with no periods in it by period and getting the first value simply returns the string unchanged. We then fall back to the undefined yaml file as expected.
• Passing a supported minor+major release: If we have a specific file for this release, it is found by the first query. Otherwise, we fall back to the major release only which gets found.
• Passing a supported major release + unsupported minor release: The first query fails but the second succeeds since we fall back to the supported major version.
• Passing an unsupported major+minor release: We search for a file containing the complete version as well as only the major; neither is found so we fall back to the undefined yaml file as expected.