Upgrade phoenix_live_view to 1.2.12 and tighten constraints

- phoenix_live_view ~1.1.0 -> ~1.2 (locked 1.2.12, latest stable)
- plug ~> 1.17 -> ~> 1.20 (1.20.3 already latest stable)
- phoenix ~> 1.8.1 -> keeps 1.8.15, also latest stable

Constraints now document the minimum secure/stable versions.

mix.exs

@@ -28,11 +28,11 @@ defmodule Microprints.MixProject do
   defp deps do
     [
       {:phoenix, "~> 1.8.1"},
-      {:phoenix_live_view, "~> 1.1.0"},
+      {:phoenix_live_view, "~> 1.2"},
       {:phoenix_html, "~> 4.1"},
       {:phoenix_pubsub, "~> 2.1"},
       {:phoenix_template, "~> 1.0"},
-      {:plug, "~> 1.17"},
+      {:plug, "~> 1.20"},
       {:plug_crypto, "~> 2.0"},
       {:telemetry, "~> 1.0"},
       {:mime, "~> 2.0"}