Add Service Templates deprecation warning banner

What does this MR do?

Issue #204785 (closed)

We are going to deprecate Service Templates in favor of a more comprehensive solution with instance-level integrations &2430 which was released in %13.3. This MR adds an info alert to make users aware of the new feature.

Screenshots

Copy

Service Templates will be soon be deprecated.
Try using the latest version of Integrations instead.

• Button 1: Go to Integration => /admin/application_settings/integrations
• Button 2: Learn more => documentation page https://docs.gitlab.com/ee/user/admin_area/settings/project_integration_management

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team