remove 2018.3 support because it is incompatible with necessary assembly definition functionality

Author: Karl Henkel

.yamato/promotion.yml

@@ -1,5 +1,4 @@
 test_editors:
-  - version: 2018.3
   - version: 2018.4
   - version: 2019.4
   - version: 2020.1

.yamato/upm-ci.yml

@@ -1,5 +1,4 @@
 test_editors:
-  - version: 2018.3
   - version: 2018.4
   - version: 2019.4
   - version: 2020.1

package.json

@@ -2,7 +2,7 @@
   "name": "com.unity.probuilder",
   "displayName": "ProBuilder",
   "version": "4.4.2",
-  "unity": "2018.3",
+  "unity": "2018.4",
   "description": "Build, edit, and texture custom geometry in Unity. Use ProBuilder for in-scene level design, prototyping, collision meshes, all with on-the-fly play-testing.\n\nAdvanced features include UV editing, vertex colors, parametric shapes, and texture blending. With ProBuilder's model export feature it's easy to tweak your levels in any external 3D modelling suite.",
   "keywords": [
     "3d",