Update Node testing versions in CI: match server v22 & drop v18

Node v18 was already officially unsupported since Mockttp v4, but we've
kept testing in it. Still working for now, but unnecessary and adds
extra delay and effort - now gone.

Author: pimterry

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x, v22.14.0, '*']
+        node-version: [20.x, 22.x, v22.20.0, '*']
 
     steps:
       - uses: actions/checkout@v4