OKR: CI/CD/Variables - settings-related UI text

Description

This MR Updates the UI Text of the variables sections of settings to follow the guidelines below:

• Instance Variables
• Group Variables
• Project Variables

Update UI text to meet our style guidelines

• Related to #299828 (closed)
• Related to technical-writing#278 (closed)

Update this section of the UI to meet the Pajamas style guidelines: Settings > CI/CD > Variables.

Remember: With UI text, less is more. If you need to go into detail, link to the docs.

Review grammar and style

At a minimum, address these issues:

• Present tense rather than future. https://design.gitlab.com/content/voice-tone#instructions
• Active voice. https://design.gitlab.com/content/voice-tone#active-voice
• No periods for settings or headings. https://design.gitlab.com/content/punctuation#periods
• Periods for all help text.
• No repetitive or unnecessary text. (Things like: This setting is... or "Use this setting to.")
• Oxford comma used correctly. (This, this, and this.)
• Language is parallel. (Review all the settings in the section to ensure they are similarly-worded.) https://design.gitlab.com/content/voice-tone#parallelism
• If a setting is optional, then "optional" is in parentheses at the end of the setting name. https://design.gitlab.com/components/form/#required-information
• Latinisms (i.e., e.g.) removed. https://design.gitlab.com/content/voice-tone#avoid-latin-abbreviations
• Don't use "allow" unless you're specifically talking about security. For example, "Allows users to fork the repo" should be "Users can fork the repo."
• Do a final review of everything you've updated and consider whether it affects the docs. If it does, open a follow-up issue or an MR to update the docs.
• If you have a link to documentation, attempt to refer to the link as a question (like, "How do I configure runners?"). If not possible, use "Learn more." https://design.gitlab.com/usability/helping-users#formatting-help-content

Screenshots

Before	After
PROJECT:
GROUP:
INSTANCE:

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

Related to #299828 (closed)