Replace tool version manager config with unified setting

What does this merge request do and why?

• Unified configuration: Replace separate mise and asdf config options with a single tool_version_manager setting
• Remove asdf support: Strip out asdf-related detection, execution logic, and fallback paths in the affected files
• Standardize on mise: Tool version management in these components now goes through mise when enabled - commands, environment setup, package installation, etc.
• Simplify method names: Rename mise_available? to tool_version_manager_available? and remove asdf_available? checks where possible
• Update error messages: PostgreSQL reinstall instructions and other user-facing messages now only reference mise commands
• Clean up tests: Remove asdf test scenarios and update existing tests to use the new mise-only approach for the modified components
• Reduce complexity: Eliminate branching logic that handled both tool managers in these areas, making the codebase simpler to maintain

This is part of an iterative migration from a dual asdf/mise system to mise-only, focusing on core dependency management, PostgreSQL handling, and rake execution components while preserving the same functionality.

See: #2917 (closed)

How to set up and validate locally

Basic functionality:

• Run gdk doctor to ensure dependency checks still work with the new tool_version_manager config
• Test GDK_SELF_UPDATE=0 gdk update to verify tool versions are updated correctly using mise
• Check that gdk start and gdk stop work without issues

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.