Improve i18n of ProjectFormGroup

What does this MR do and why?

Extract and improve i18n strings

• Extract strings to an i18n object in the component's options
• Refactor the "fork this project" string so as not to break up the sentence, to better conform to our guidelines.

Part of #241801 (closed).

Screenshots or screen recordings

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There is no visual change, but here's what it looks like anyway:

How to set up and validate locally

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

No forks case

1. Find or create a public project with no private forks
2. Find or create a confidential issue in the project
3. Open the Create confidential merge request dropdown on the confidential issue's page

Forks case

1. Find or create a public project
2. Create one or more private forks of that project
3. Find or create a confidential issue in the original project
4. Open the Create confidential merge request dropdown on the issue page

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 #241801 (closed)