chore(documentation): remove ng-bootstrap custom select (#5302)

## 📄 Description

Remove documentation about the ng-bootstrap custom select.

Note that the changeset is set to `minor` as it's only the documentation
that has been updated. Technically, all the styles are still present as
they are common with the `form-select`, and the JS comes from importing
ng-bootstrap which we have not removed.

Author: leagrdv

.changeset/three-lizards-fail.md

@@ -0,0 +1,5 @@
+---
+'@swisspost/design-system-documentation': patch
+---
+
+Removed the deprecated ng-bootstrap custom select documentation page.

packages/documentation/src/stories/components/select-custom/select-custom-basic.sample.html

@@ -602,6 +602,12 @@ export class MigrationV89Component extends LitElement {
                     <span class="tag tag-sm tag-danger">breaking</span>
                   </p>
                 </li>
+                <li class="mb-16">
+                  <p>
+                    The ng-bootstrap <code>custom select</code> has been removed
+                    <span class="tag tag-sm tag-danger">breaking</span>
+                  </p>
+                </li>
                 <li class="mb-16">
                   <p>
                     The ng-bootstrap <code>carousel</code> component has been removed