Align v7 feature selectors and legacy lifecycle

[arturcic]

Summary

Align the v7 feature selectors with the implementations that shipped, log each
resolved selection, and remove the v6 parser/configuration compatibility paths
on their approved lifecycle. Keep the Git backend lifecycle tied to the
managed-Git roadmap rather than removing libgit2 prematurely.

Blocked by #5134.

Environment variables

Concern	Environment variable	Values	v7.0 default
Argument parser	GITVERSION_ARGUMENT_PARSER_VERSION	v6, v7	v7
Configuration structure	GITVERSION_CONFIGURATION_VERSION	v6, v7	v7
Git backend	GITVERSION_GIT_BACKEND	libgit2, managed	libgit2

Replace:

GITVERSION_USE_V6_ARGUMENT_PARSER=true

with:

GITVERSION_ARGUMENT_PARSER_VERSION=v6

The old boolean variable is removed rather than supported as an alias. If it is
still set, GitVersion fails with a clear message identifying
GITVERSION_ARGUMENT_PARSER_VERSION=v6 as its replacement.

v7 behavior

When the variables are unset in v7.0:

• Use the POSIX v7 argument parser.
• Use the nested v7 calculation/output configuration.
• Use the libgit2 backend. The managed backend remains opt-in until its own
  readiness roadmap changes the default.

Temporary compatibility selections:

GITVERSION_ARGUMENT_PARSER_VERSION=v6
GITVERSION_CONFIGURATION_VERSION=v6

The Git backend selector remains independent:

GITVERSION_GIT_BACKEND=libgit2
GITVERSION_GIT_BACKEND=managed

Selector behavior:

• Values are trimmed and case-insensitive.
• Empty values behave as unset.
• Unknown values fail fast and list the accepted values.
• Selection is centralized so all applicable composition roots behave
  consistently.
• The selectors are independent.

Legacy lifecycle

• Remove the legacy argument parser and flat v6 configuration runtime support
  on the approved v7.x lifecycle while retaining gitversion config migrate.
• Keep retired parser/configuration selector values recognizable long enough to
  emit actionable diagnostics.
• Do not remove LibGit2Sharp or its selector in this issue. Its default flip,
  soak period, and removal remain governed by the managed-Git roadmap.
• Remove feature-selector environment variables #5136 tracks major-version cleanup of compatibility selectors after their
  prerequisites are complete.

Selection logging

Log all resolved selections at information level, using the actual effective
values, for example:

Argument parser version: v7
Configuration version: v7
Git backend: libgit2

• Log the effective selection whether it came from the default or an
  environment variable.
• Do not include these messages in machine-readable JSON, YAML, or
  single-variable stdout.
• Make configuration-version logging available to non-CLI entry points such as
  MSBuild where normal logging exists.
• Do not log credentials or unrelated environment values.

Documentation sweep

Replace the remaining GITVERSION_USE_V6_ARGUMENT_PARSER references when the
new selector is implemented:

• BREAKING_CHANGES.md
• docs/input/docs/migration/v6-to-v7.md (migration warning and environment
  variable table)
• docs/input/docs/usage/cli/arguments.md (selector introduction and
  config migrate availability)

Acceptance criteria

• Tests cover unset, empty, case-insensitive, whitespace-padded, valid, and
  invalid values.
• v7.0 defaults select the v7 parser, v7 configuration, and libgit2 backend.
• Explicit parser/configuration fallbacks remain functional until their
  approved removal point.
• GITVERSION_USE_V6_ARGUMENT_PARSER is rejected with its replacement.
• All resolved selections are logged without contaminating machine-readable
  stdout.
• Parser/configuration legacy removal produces actionable diagnostics and does
  not remove gitversion config migrate.
• Git backend behavior and documentation stay aligned with the managed-Git
  implementation roadmap.
• Documentation and migration guidance use the aligned selector names.

[arturcic]

Follow-up for the next major release: #5136 removes the selector environment variables entirely after the v7.1 legacy implementation cleanup.