Update feature proposal template with "Intended users"

What does this MR do?

Changes the "Target audience" section of the Feature Proposal issue template to "Intended users" and expands the section to include more general groups of users for which we might not have relevant personas. This change reflects an action item from the UX and Product Marketing meeting on Personas. Reasoning for the changes:

1. Existing personas are not up to date and don't cover all of our users
2. Requiring a persona field to open a feature proposal can be a blocker when there isn't a relevant persona
3. Having the option to leave the field defined by the creator, empty, or "unknown" is less prescriptive and enables exploration during product discovery

This MR also replaces the list of personas links and labels with a single link to the personas page because:

1. The list of links made the template unnecessarily long
2. This template exists in multiple projects, and any time individual personas and personas links change, the templates become obsolete

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer