feat: Add $CODE_QUALITY_IMAGE_TAG

What does this MR do and why?

Updates the Code Quality template to use $CODE_QUALITY_IMAGE_TAG. Also sets it to 0.87.3, which is the current version.

This allows for easier overriding of the image version.

Documentation of $CODE_QUALITY_IMAGE_TAG will be taken care of with Document CODE_QUALITY_IMAGE_TAG (gitlab-org/ci-cd/codequality!68 - merged)

Relevant issue: A new version (v0.87.3) is available. (gitlab-org/ci-cd/codequality#63 - closed)

Example MR using the changes to this template: rossfuhrman/insecure!2 (diffs)

MR acceptance checklist

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