Implement versioned calculation and output configuration

[arturcic]

Parent: #5131

Follows the release-selector matrix defined by #5135.

Summary

Implement the nested calculation and output configuration model, with separate calculation/output branch sections and version-aware runtime selection.

Configuration structure

• Add explicit calculation and output configuration models.
• Move all settings according to the ownership defined by [Feature] Group configuration under calculation and output sections with migration support #5131.
• Split branch settings between calculation.branches and output.branches.
• Put custom-version-format and pre-release-weight under output defaults and output.branches.
• Merge corresponding calculation/output branch entries into effective configuration.
• Allow output entries for branches supplied by built-in workflows.

Release selection

Implement GITVERSION_CONFIGURATION_VERSION=v6|v7 consistently across CLI, MSBuild, and other entry points:

• v7.0: unset/empty defaults to v7; explicit v6 enables the temporary flat-format fallback.
• v7.1: remove v6 runtime loading; v6 produces an actionable migration error and explicit v7 is accepted as a no-op.
• Values are trimmed and case-insensitive; invalid values fail fast.
• Detect flat, nested, mixed, empty, and selected-format mismatches.
• Structure the compatibility validator for removal in v8 by Remove feature-selector environment variables #5136.

Configuration behavior

• Both formats resolve to equivalent effective configuration in v7.0.
• Convert built-in workflows and preserve workflow/user precedence.
• Update builders, inheritance, serialization, --show-config, cache inputs, and public API baselines.

Version-aware --override-config

• v7 paths include calculation.tag-prefix, calculation.branches.main.increment, output.update-build-number, and output.branches.main.pre-release-weight.
• v6 mode retains flat override paths.
• Reject paths from the wrong version with the correct replacement when known.
• Validate all overrides before applying any and never translate silently.
• Point users to gitversion config migrate where appropriate.

Acceptance criteria

• Every existing setting has the destination specified by [Feature] Group configuration under calculation and output sections with migration support #5131.
• v6/v7 parity, workflow merging, selectors, document detection, invalid values, and removal behavior are tested.
• Override validation is tested in both modes.
• --show-config, caching, CLI, MSBuild, builders, serialization, and API baselines are updated.