Remove support for kernels older than 4.18

.checkpatch.conf

@@ -0,0 +1,3 @@
+# The input-wacom paths are not from upstream. Do not check them.
+
+--ignore FILE_PATH_CHANGES

.github/workflows/checkpatch.yml

@@ -1,8 +1,7 @@
 on:
   pull_request:
     paths:
-    - '3.17/**'
-    - '4.5/**'
+    - '4.18/**'
 
 permissions:
   contents: read

.github/workflows/main.yml

@@ -12,38 +12,18 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        kernel: ["6.3", "5.10", "4.19", "4.14", "4.10", "4.6", "4.5", "4.2", "4.1", "3.19", "3.18", "3.17"]
+        kernel: ["6.3", "5.15", "5.10", "5.4", "5.0", "4.19"]
         include:
-          - kernel: "3.17"
-            compile_cflags: -fno-pie -Wno-error=format-truncation
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "3.18"
-            compile_cflags: -fno-pie -Wno-error=format-truncation
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "3.19"
-            compile_cflags: -fno-pie -Wno-error=format-truncation
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.1"
-            compile_cflags: -fno-pie -Wno-error=format-truncation
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.2"
-            compile_cflags: -fno-pie -Wno-error=format-truncation
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.5"
-            compile_cflags: -fno-pie -Wno-error=format-truncation -Wno-error=pointer-sign
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.6"
-            compile_cflags: -fno-pie -Wno-error=format-truncation -Wno-error=pointer-sign
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.10"
-            compile_cflags: -fno-pie -Wno-error=format-truncation -Wno-error=pointer-sign
-            prepare_cflags: -fno-pie -no-pie
-          - kernel: "4.14"
-            compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
           - kernel: "4.19"
             compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
+          - kernel: "5.0"
+            compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
+          - kernel: "5.4"
+            compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
           - kernel: "5.10"
             compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
+          - kernel: "5.15"
+            compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
           - kernel: "6.3"
             compile_cflags: -Wno-error=format-truncation -Wno-error=pointer-sign
             build_makeflags: KBUILD_MODPOST_WARN=1