feat(node-versions): dropped support for node v18 and raised the minimum for v20

reasoning for the chosen version:
https://github.com/semantic-release/gitlab/pull/647#discussion_r1423131205

BREAKING CHANGE: support for node v18 has been dropped and the minimum for v20 is now v20.8.1

related to https://github.com/semantic-release/semantic-release/discussions/3088

.github/workflows/test.yml

@@ -25,9 +25,9 @@ jobs:
     strategy:
       matrix:
         node-version:
-          - 18.17.0
-          - 20.6.1
+          - 20.8.1
           - 20
+          - 21
 
     runs-on: ubuntu-latest
     timeout-minutes: 5

package-lock.json

@@ -67,7 +67,7 @@
         "testdouble": "3.20.1"
       },
       "engines": {
-        "node": "^18.17 || >=20.6.1"
+        "node": ">=20.8.1"
       }
     },
     "node_modules/@babel/code-frame": {

package.json

@@ -82,7 +82,7 @@
     "testdouble": "3.20.1"
   },
   "engines": {
-    "node": "^18.17 || >=20.6.1"
+    "node": ">=20.8.1"
   },
   "files": [
     "bin",