Pin API Fuzzing version to 1.6

What does this MR do?

Set API Fuzzing version to "1.6" instead of "latest". This change is being made to ensure future releases of the API Fuzzing container will not break GitLab v13 offline users.

Addresses #327510 (closed)

CI Template

• This change is backwards compatible. The current API Fuzzing container version is 1.6.XXX.

• No tests have been created of modified because the CI template logic has not changed

• Test Project:

  • Project
  • CI Template
  • Job

  

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• [-] Documentation (if required)
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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

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• [-] 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
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