Remove feature-selector environment variables

[arturcic]

Summary

Remove the v7 compatibility selector environment variables in GitVersion v8 after their legacy implementations have been removed in v7.1.

This is the final cleanup following the v7 feature-selector alignment issue.

Remove

• GITVERSION_ARGUMENT_PARSER_VERSION
• GITVERSION_CONFIGURATION_VERSION
• GITVERSION_GIT_BACKEND
• Selector types, parsing, validation, logging branches, compatibility diagnostics, and tests that exist only for these variables.

GitVersion v8 has one implementation for each concern:

• POSIX argument parser
• Nested calculation/output configuration
• Managed Git backend

Register and use those implementations directly rather than passing through feature-selector abstractions.

Behavior

• The three retired variables no longer influence execution.
• Prefer failing fast with a clear removal message when a retired variable is set, rather than silently accepting stale deployment configuration.
• Remove the per-selector informational log lines where they no longer communicate a choice. Retain ordinary diagnostic information where useful, such as identifying the managed Git implementation.
• Keep gitversion config migrate available for converting old configuration files; migration support does not imply runtime support for the v6 configuration format.

Acceptance criteria

• No production selection code reads the three variables.
• No compatibility selector abstractions or no-op selection branches remain.
• Packaging contains no LibGit2Sharp/native backend artifacts.
• Legacy argument parsing and legacy configuration loading remain unavailable.
• Tests verify actionable diagnostics when retired variables are supplied.
• v8 breaking-change and migration documentation lists the removed variables and fixed implementations.