[api-minor] Update the minimum supported Node.js version to 20

This patch updates the minimum supported environments as follows:
 - Node.js 20, which was released on 2023-04-18 and has now entered the "Maintenance"-phase; see https://github.com/nodejs/release#release-schedule

Furthermore, note also that Node.js 18 will fairly soon reach EOL.

gulpfile.mjs

@@ -84,7 +84,7 @@ const ENV_TARGETS = [
   "Chrome >= 103",
   "Firefox ESR",
   "Safari >= 16.4",
-  "Node >= 18",
+  "Node >= 20",
   "> 1%",
   "not IE > 0",
   "not dead",
@@ -2271,7 +2271,7 @@ function packageJson() {
       url: `git+${DIST_GIT_URL}`,
     },
     engines: {
-      node: ">=18",
+      node: ">=20",
     },
     scripts: {},
   };