Update README.md with new prefers-color-scheme media feature.

The old GitHub method using #gh-dark-mode-only and #gh-light-mode-only
is deprecated and will stop working

Author: georgeguimaraes

Diff for: .markdownlint.jsonc

@@ -26,6 +26,8 @@
       "a",
       "br",
       "img",
+      "picture",
+      "source",
       "noscript",
       "p",
       "script"

Diff for: README.md

@@ -1,5 +1,9 @@
-<h1><img src="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo.png#gh-light-mode-only" width="200" alt="Elixir">
-<img src="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo-dark.png#gh-dark-mode-only" width="200" alt="Elixir"></h1>
+<h1>
+ <picture>
+  <source media="(prefers-color-scheme: dark)" srcset="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo-dark.png">
+  <img alt="Elixir logo" src="https://github.com/elixir-lang/elixir-lang.github.com/raw/main/images/logo/logo.png" width="200">
+ </picture>
+</h1>
 
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