Added Canvas content to Validation issue template

What does this MR do?

This merge request adds the content from the Opportunity Canvas to an issue template. This MR is intended to deprecate the use of the Mural template in https://about.gitlab.com/handbook/product-development-flow/#problem-validation-process-description in favor of GitLab issues. This is for several reasons:

• No reason to use Mural. Mural is great for unstructured brainstorming and collaborative design sessions, but the current canvas does not benefit from any of the benefits of the application: it's not worked on synchronously and is highly structured in terms of how it's intended to be filled out.
• It's painful to fill out in Mural. Hyperlinks are painful to add (you can't add multiple links to a single text box), it's hard to edit/modify text (need to double-click on a text box to make it editable, which also moves things around on the canvas; there's also no text cursor when editing text), and fonts sometimes get small and hard to read.
• Easier to iterate on in GitLab. If we get asynchronous feedback on a design in an issue, the only way of editing it is to re-open Mural and make the change, then re-export the canvas as an image back into GitLab. If it's an issue, we can just edit the issue and spend less time managing tools.
• Having more content in GitLab makes it easier to dogfood and allow others to contribute.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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