chore(regression): remove dead test_advanced_config + Greenplum config gitignore (#9949)

Two related bits of long-stale plumbing in web/regression/:

* test_advanced_config.json.in — the README has told users for years to
  copy this template to test_advance_config.json (no "d") and customise
  it, but a repo-wide grep for either spelling shows zero code, test,
  or CI references. The .in template is dead, and the README/.gitignore
  also disagree about whether the copied filename has a "d" in it,
  which is a typo trap that has gone unnoticed precisely because nobody
  actually performs the copy step.

* test_greenplum_config.json — Greenplum support was removed from
  pgAdmin years ago (only references left in the tree are historical
  release-notes entries from versions 1.4 through 4.12). Nothing reads
  the config any more.

Drop the dead template, prune both stale lines from
web/regression/.gitignore, and simplify the README to describe just
the server-side test_config.json that the framework actually consumes.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>