Docs: Remove reference to removed variables

What does this MR do?

This MR removes references to two variables that are no longer used by the gitleaks tool that Secret Detection relies on.

In #243564 (closed) we:

Update template Secret-Detection.gitlab.yml to use SECRET_DETECTION_COMMITS instead of SECRET_DETECTION_COMMIT_FROM/TO

Setting the SECRET_DETECTION_COMMIT_FROM and SECRET_DETECTION_COMMIT_TO variables as instructed in the docs will have no effect.

This MR updates the docs to remove the usage of those variables as requested in this issue. This came to my attention while working on a ticket that is available to GitLab team members with access to Zendesk. 🎫

There's more information about my findings in the Docs: Update Secret Detection documentation to reflect move away from SECRET_DETECTION_COMMIT_FROM and SECRET_DETECTION_COMMIT_TO in favor of SECRET_DETECTION_COMMITS issue. I'm planning an accompanying MR with notes about using SECRET_DETECTION_COMMITS.

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.