Shutoff spellcheck on CI variables

What does this MR do and why?

spellcheck=false directs browsers not to spell check fields. All sensitive fields should have spellcheck=false since browsers like Chrome and Edge may send data in those fields to the cloud for spell check unless it is explicitly turned off.

Addresses: https://gitlab.com/gitlab-org/gitlab/-/issues/375380

Screenshots or screen recordings

Before:

After:

How to set up and validate locally

Go to a project, Settings > CI/CD > Variables Click "Add Variable" Type in a misspelled word in the value field. You should not see red underscores.

MR acceptance checklist

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