Remove deprecated CLI flags in v2

[thomhurst]

Summary

Remove CLI flags deprecated during TUnit v1 as part of the v2 breaking-change cleanup.

Known flags

• --report-html: now a no-op because TUnit generates HTML reports automatically. Use TUNIT_DISABLE_HTML_REPORTER=true to disable report generation.

Audit all ICommandLineOptionsProvider implementations and documentation for any other deprecated CLI aliases before v2 ships.

Acceptance criteria

• Remove deprecated CLI option registrations and handling.
• Remove obsolete warnings and compatibility code.
• Update command-line reference and migration documentation.
• Add v2 migration notes listing each removed flag and its replacement.
• Add or update tests confirming removed flags are no longer registered.

Related: #6677.

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