Fix image links in Modrinth description

Modrinth proxies images hosted on non-trusted domains through wsrv.nl,
for understandable reasons. However, wsrv.nl blocks tweaked.cc - I'm not
sure why. Instead we reference the image on GH directly, which works!

Also:
 - Fix the modrinthSyncBody task pointing to a missing file.
 - Update the licenses of a few files, post getting permission from
   people. <3 all.

projects/core/src/test/resources/test-rom/spec/apis/window_spec.lua

@@ -1,6 +1,6 @@
 -- SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
 --
--- SPDX-License-Identifier: LicenseRef-CCPL
+-- SPDX-License-Identifier: MPL-2.0
 
 describe("The window library", function()
     local function mk()

projects/core/src/test/resources/test-rom/spec/programs/turtle/craft_spec.lua

@@ -1,6 +1,6 @@
 -- SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
 --
--- SPDX-License-Identifier: LicenseRef-CCPL
+-- SPDX-License-Identifier: MPL-2.0
 
 local capture = require "test_helpers".capture_program