Updated the GitHub issue link to open in a new tab

[josevarghese]

What is this PR doing?

Due to the Content Security Policy, the link to the GitHub issue does not open within the Playground 'Add new plugin' and 'Add new theme' pages. To fix this, add the target attribute to load the link in a new tab.

What problem is it solving?

The error occurs when someone clicks on the "experimental, opt-in feature" link within the Playground due to the CSP.

How is the problem addressed?

Added the target attribute to the hyperlink to load the link in a new tab.

Testing Instructions

1. Open playground
2. Navigate to the Plugins > Add New Plugins or Appearance > Themes > Add New Theme (button)
3. You can notice the admin error notice mentioning "Network access is an experimental, opt-in feature, which means you need to enable it to allow Playground to access the Plugins/Themes directories."
4. Click on the "experimental, opt-in feature" hyperlink, where you can notice the GitHub issue link is not loaded due to the CSP.

[adamziel]

Thank you so much @josevarghese!