docs: update README environment variables

Description

Follow-up to !2347 (merged), which deprecated the non-prefixed environment variables (DEBUG, FORCE_HYPERLINKS, GLAMOUR_STYLE, NO_PROMPT) in favor of GLAB_-prefixed equivalents.

Updates the README.md to reflect the deprecation:

• Adds GLAB_DEBUG, GLAB_FORCE_HYPERLINKS, GLAB_GLAMOUR_STYLE, and GLAB_NO_PROMPT as primary entries in the glab configuration variables table, with their config.yml equivalents and default values.
• Replaces the original rows for FORCE_HYPERLINKS, GLAMOUR_STYLE, and NO_PROMPT with deprecated entries pointing to the new names.
• Moves GLAB_DEBUG from the "Other variables" table to "glab configuration variables", where it belongs alongside the other GLAB_-prefixed variables.
• Updates the DEBUG row in "Other variables" to a deprecated entry.
• Updates the "Debugging" section to reference GLAB_DEBUG.

Related issues and merge requests

• Prefix environment variables used by glab with ... (#7965 - closed)
• docs: update env vars table to use GLAB_ prefix... (!3348 - merged)

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.