Update Ruby versions (#93)

Automatically created pull-request to update Ruby and Rubygems versions.

This is the result of running:

```
nix run .#update
```

See the workflow here:
https://github.com/bobvanderlinden/nixpkgs-ruby/blob/master/.github/workflows/update.yml

rubygems/versions.json

@@ -395,13 +395,17 @@
     "3.4.19": {
       "url": "https://github.com/rubygems/rubygems/archive/refs/tags/v3.4.19.tar.gz",
       "hash": "sha256-ntXPB8C514mpKBhaRXIrQyToz2Zz/9e3bQClixxBBzc="
+    },
+    "3.4.20": {
+      "url": "https://github.com/rubygems/rubygems/archive/refs/tags/v3.4.20.tar.gz",
+      "hash": "sha256-pnXPuXWceELp4n14ZUu/h23AYbdA1z8wLMY7QYGoojY="
     }
   },
   "aliases": {
     "1": "1.8.30",
     "2": "2.7.6",
-    "3": "3.4.19",
-    "3.4": "3.4.19",
+    "3": "3.4.20",
+    "3.4": "3.4.20",
     "3.3": "3.3.26",
     "3.2": "3.2.34",
     "3.1": "3.1.4",
@@ -411,7 +415,7 @@
     "2.2": "2.2.3",
     "2.0": "2.0.15",
     "1.8": "1.8.30",
-    "3_4": "3.4.19",
+    "3_4": "3.4.20",
     "3_3": "3.3.26",
     "3_2": "3.2.34",
     "3_1": "3.1.4",