remove ubuntu 18.04 from ci, no resources for this on github
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ed1f322..03e4151 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@
     runs-on: "${{ matrix.os }}"
     strategy:
       matrix:
-        os: [ubuntu-22.04, ubuntu-20.04, ubuntu-18.04]
+        os: [ubuntu-22.04, ubuntu-20.04]
     env:
       AFL_SKIP_CPUFREQ: 1
       AFL_I_DONT_CARE_ABOUT_MISSING_CRASHES: 1