Commit to requiring `ruby@3.1`

Remove `ruby@2.6`, `ruby@2.7`, `ruby@3.0`, `rails@6.1`, and `rails@7.0`
from the CI matrix, along with any `RUBY_VERSION < "3"` conditionals.

The implementation changes include replace older syntaxes with newer
variants. For example, utilize `...` in place of `*args, &block`, rely
on `**`-ing attribute `Hash` arguments, etc.

The Background
---

Consider an application with a shared `application/flash` partial that
accepts its message as a partial-local assignment:

```erb
<%# app/views/application/_flash.html.erb %>

<p role="alert"><%= message %></p>
```

Also consider an `application/layout.turbo_stream.erb` template for
appending the `Flash` messages to a element with `[id="flashes"]`:

```erb
<%# app/views/layouts/application.turbo_stream.erb %>

<% flash.each do |name, message| %>
  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
<% end %>
```

This works as you'd expect, since the `:partial` and `:locals` keyword
arguments are forwarded along to the underlying `render` call.

The Scenario
---

Now consider that the `application/flash` message changed its interface
to expect the `message` as block content yielded to the partial instead
of an assignment to the `:locals` options:

```diff
 <%# app/views/application/_flash.html.erb %>

-<p role="alert"><%= message %></p>
+<p role="alert"><%= yield %></p>
```

The `layouts/application.turbo_stream.erb` template would need to change
as well:

```diff
 <%# app/views/layouts/application.turbo_stream.erb %>

 <% flash.each do |name, message| %>
-  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
+  <%= turbo_stream.append "flashes", partial: "application/flash" do %>
+    <span style="color: red"><%= message %></span>
+  <%= end %>
 <% end %>
```

The Problem
---

This style of invocation of `turbo_stream.append` does not work the same
as if it were passed a block of content generated by calling `render`
with the same keywords.

The presence of a `&block` argument triggers an entirely separate code
path than the presence of the `**rendering` keywords.

To work around this issue, you'd have to capture the rendering
separately:

```diff
 <%# app/views/layouts/application.turbo_stream.erb %>

 <% flash.each do |name, message| %>
-  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
+  <% content = capture do %>
+    <%= render partial: "application/flash" do %>
+      <span style="color: red"><%= message %></span>
+    <% end %>
+  <% end %>
+
+  <%= turbo_stream.append "flashes", content %>
 <% end %>
```

The Proposal
---

This commit alters the tag builder's decision making process to
incorporate a check for a combination of both a `&block` and `:partial`
or `:layout` keyword arguments.

This reverts commit 0e42702.