Add CHANGELOG entry to add bigdecimal as Active Support dependency

Follow up rails#49152
rails#49039

- `bigdecimal_to_d.rb`
```
require 'bigdecimal/util'
p "Ruby version: #{RUBY_VERSION}"
p "BigDecimal version: #{BigDecimal::VERSION}"
p "123.455.to_d(5): #{123.455.to_d(5)}"
```

- Ruby 2.7 and 3.0 rounds down
```
$ ruby bigdecimal_to_d.rb
"Ruby version: 2.7.8"
"BigDecimal version: 2.0.0"
"123.455.to_d(5): 0.12345e3"
```

```
$ ruby bigdecimal_to_d.rb
"Ruby version: 3.0.6"
"BigDecimal version: 3.0.0"
"123.455.to_d(5): 0.12345e3"
```

- Ruby 3.1 and 3.2 rounds up

```
$ ruby bigdecimal_to_d.rb
"Ruby version: 3.1.4"
"BigDecimal version: 3.1.1"
"123.455.to_d(5): 0.12346e3"
```

```
$ ruby bigdecimal_to_d.rb
"Ruby version: 3.2.2"
"BigDecimal version: 3.1.3"
"123.455.to_d(5): 0.12346e3"
```

activesupport/CHANGELOG.md

@@ -4,6 +4,14 @@
 
 ## Rails 7.1.0.beta1 (September 13, 2023) ##
 
+*   Add `bigdecimal` as Active Support dependency that is a bundled gem candidate for Ruby 3.4.
+
+    `bigdecimal` 3.1.4 or higher version will be installed.
+    Ruby 2.7 and 3.0 users who want `bigdecimal` version 2.0.0 or 3.0.0 behavior as a default gem,
+    pin the `bigdecimal` version in your application Gemfile.
+
+    *Koichi ITO*
+
 *   Add `drb`, `mutex_m` and `base64` that are bundled gem candidates for Ruby 3.4
 
     *Yasuo Honda*