[gha] switch ubuntu to 18.04

Signed-off-by: R4SAS <r4sas@i2pmail.org>

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build-make:
     name: Make with USE_UPNP=${{ matrix.with_upnp }}
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: true
       matrix:
@@ -21,7 +21,7 @@ jobs:
       run: make USE_UPNP=${{ matrix.with_upnp }} -j3
   build-cmake:
     name: CMake with -DWITH_UPNP=${{ matrix.with_upnp }}
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: true
       matrix: