chore(deps): upgrade to Rails 7.2.3.2, Ruby 3.4.10 and latest gems

Upgrades Rails from 7.1.6 to 7.2.3.2, Ruby from 3.4.6 to 3.4.10, and
brings the rest of the bundle up to date.

Two things were broken by Rails 7.2 and are fixed here:

* MigrationWaiter called `ActiveRecord::Base.connection.migration_context`,
  which moved to the connection pool in 7.2. This raised NoMethodError on
  boot for any deployment with `migration_waiter.enabled` set, and had no
  test covering it.
* `config.action_dispatch.show_exceptions = false` was deprecated in 7.1
  and removed in 7.2. Now `:none`.

`json` is pinned to < 3. Active Support 7.2 passes `quirks_mode:` to both
`JSON.generate` and `JSON.parse`, and json 3.0 removed that option, so an
unpinned json resolves to 3.x and makes every `to_json` raise - webhook
payloads included. Nothing else in the bundle constrains it. The pin can go
when we reach Rails 8.1, which no longer passes the option.

`dynamic_form` is dropped. It is unmaintained and from 1.3.1 requires Ruby
< 3.4, so it could never be updated again. Ten views used it, so
`error_messages_for` now lives in ErrorMessagesHelper and `f.error_messages`
in a form builder extension. The rendered markup is unchanged apart from a
heading and description the stylesheet had always hidden; the rules that
hid them are removed too.

`execjs` is unpinned. The `< 2.8` constraint dated from the Rails 6.0
upgrade and asset output is byte-identical without it.

Notable gem majors: puma 8, haml 7, secure_headers 7, sentry 7, jwt 3,
prometheus-client 5, highline 3, omniauth-rails_csrf_protection 2,
rspec-rails 8, shoulda-matchers 8, rubocop 1.91. secure_headers 7 changes
X-XSS-Protection from "1; mode=block" to "0", which is the correct modern
default and matches Rails' own.

`config.load_defaults` deliberately stays at 7.0. The pending 7.1 and 7.2
options are listed, commented out, in new_framework_defaults_7_1.rb and
_7_2.rb so they can be adopted in stages - several change the signed cookie
and cache formats and need a staged deploy. The vestigial
new_framework_defaults_7_0.rb is removed, as `load_defaults 7.0` already
applies all of it.

Adds tests for the areas this touched, which were mostly uncovered:
MigrationWaiter, the serialized columns that gate a defaults bump, JSON
encoding, HAML template compilation for all 103 templates, page rendering,
form error rendering and security headers. The assets Gemfile group now
includes :test so the view layer can be exercised at all.

spec/lib/json_encoding_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+# Active Support 7.2 calls `JSON.generate(..., quirks_mode: true)` and
+# `JSON.parse(json, quirks_mode: true)`. The json gem removed `quirks_mode` in
+# 3.0, so an unpinned json resolves to 3.x and every `to_json` in the app raises
+# ArgumentError - webhook payloads and JSON API responses included. The Gemfile
+# pins json to < 3 because of this; these examples fail loudly if that pin is
+# removed before we are on a Rails version that no longer passes the option.
+RSpec.describe "JSON encoding" do
+  it "encodes a hash" do
+    expect({ "a" => 1 }.to_json).to eq '{"a":1}'
+  end
+
+  it "encodes nested structures with mixed types" do
+    payload = { "event" => "MessageSent", "timestamp" => 1.5, "payload" => { "list" => [1, "two", nil, true] } }
+    expect(JSON.parse(payload.to_json)).to eq payload
+  end
+
+  it "round trips through Active Support's encoder" do
+    payload = { "uuid" => "abc-123", "nested" => { "x" => [1, 2] } }
+    expect(ActiveSupport::JSON.decode(ActiveSupport::JSON.encode(payload))).to eq payload
+  end
+
+  it "encodes a bare value, which is what quirks_mode allowed" do
+    expect(ActiveSupport::JSON.encode("hello")).to eq '"hello"'
+    expect(ActiveSupport::JSON.encode(42)).to eq "42"
+  end
+
+  it "encodes times the way the webhook payload relies on" do
+    expect(Time.at(1_700_000_000).utc.to_f.to_json).to eq "1700000000.0"
+  end
+end