Make skip trial copy consistent with itself & the account dropdown

What does this MR do?

Makes the “skip trial” copy consistent with itself and with the account dropdown menu.

Screenshots

For reference, here’s the wording in the account drop-down menu:

“Start an Ultimate trial 🚀”

This MR’s Changes

Before	After
no glm_source param or glm_source other than gitlab.com “…by clicking on your avatar and choosing 'Start a free trial'”	no glm_source param or glm_source other than gitlab.com “…by selecting your avatar and choosing 'Start an Ultimate trial'”
glm_source=gitlab.com param “…by selecting your avatar and choosing 'Start an Ultimate trial'” (no difference)	glm_source=gitlab.com param “…by selecting your avatar and choosing 'Start an Ultimate trial'” (no difference)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it’s a simple copy change.
• [-] 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

This is a follow-up from #215852 (closed)