chore(SA-675): update Node.js to LTS 18.20.0

Node 15.12.0 was EOL. Updated to Node 18 LTS for security and
compatibility. Major npm package updates (octokit, jest, eslint)
require careful testing due to breaking API changes and should
be addressed in a follow-up PR.

Author: gjarzebak95

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
 
       - uses: actions/setup-node@64ed1c7eab4cce3362f8c340dee64e5eaeef8f7c # v3.6.0
         with:
-          node-version: 15.12.0
+          node-version: 18.20.0
 
       - uses: actions/cache@88522ab9f39a2ea568f7027eddc7d8d8bc9d59c8 # v3.3.1
         with:

.nvmrc

@@ -1 +1 @@
-16.20.0
+18.20.0