LICENSE: strip project header so GitHub's licensee detects GPL-3.0

The previous commit added a 5-line project header above the canonical
GPL-3.0 text:

  ProxMenux — An Interactive Menu and Web Dashboard for Proxmox VE
  Copyright (c) 2025 MacRimi

  This program is licensed under the GNU General Public License v3.0.
  The full text of the license follows.

That header (~270 bytes in a 35 KB file) pushed the file below the
Sørensen-Dice similarity threshold GitHub's licensee gem uses to
fingerprint licenses, so the repo kept showing "License not
identifiable by GitHub" (API: spdx_id = "NOASSERTION", key = "other")
even though the canonical text was right below.

Strip the header. LICENSE is now byte-exact to the FSF reference:
https://www.gnu.org/licenses/gpl-3.0.txt
(SHA256: 3972dc9744f6499f0f9b2dbf76696f2ae7ad8af9b23dde66d6af86c9dfb36986)

The project copyright still lives where GPL-3.0 expects it — in each
source file's header (as enforced by CONTRIBUTING.md's "Script Header
Template" section). LICENSE is the legal reference document; the
per-file copyright notice is the project-specific declaration.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

LICENSE

@@ -1,9 +1,3 @@
-ProxMenux — An Interactive Menu and Web Dashboard for Proxmox VE
-Copyright (c) 2025 MacRimi
-
-This program is licensed under the GNU General Public License v3.0.
-The full text of the license follows.
-
                     GNU GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007