Administration: Add iframe support in plugin installation and adjust CSS

[sahil-gidwani]

Track ticket: https://core.trac.wordpress.org/ticket/45853

This PR addresses an issue where embedded videos in a plugin’s readme.txt file do not load when searching for plugins within a WordPress install or when viewing the plugin details modal.

• Videos embedded in a plugin’s readme.txt file display correctly on WordPress.org but do not appear within the WP-Admin plugin search or the "View Details" modal.
• Inspecting the element showed an empty <span class="embed-youtube"></span>, indicating that the embedded video was stripped during sanitization.

Root Cause

• In plugin-install.php, the function install_plugin_information() sanitizes the plugin data using wp_kses().
• The $plugins_allowedtags array, which defines the permitted HTML tags, did not include support for <iframe>, causing the embedded video to be removed.

Proposed Solution

• Added support for the <iframe> tag within $plugins_allowedtags, allowing embedded videos from sources like YouTube and Vimeo to be retained.
• Applied CSS rules to ensure iframes are properly sized within the plugin details view for better UI consistency.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props sahilgidwani.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.