Replace default_value_for on HttpIntegration model

What does this MR do and why?

Related to #378997 (closed)

It replaces the usage of default_vaule_for on the AlertManagement::HttpIntegration model with a native Rails feature because that gem is unmaintained and it's blocking us from upgrading to Rails 7.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #378997 (closed)