Change _VERSION_TAG to _IMAGE_SUFFIX in API Sec templates

What does this MR do and why?

Change the variables [FUZZAPI|DAST_API]_VERSION_TAG to [FUZZAPI|DAST_API]_IMAGE_SUFFIX. This change is occurring due to reviewer feedback on a related MR from a different team.

The _VERSION_TAG variable was introduced last week via !84312 (merged). This variable name was being used across multiple teams in sec-section. However, after the MR was accepted the name was changed due to reviewer feedback on a related MR from another team in sec-section. This new MR updates the variable name to match.

• Change _VERSION_TAG to _IMAGE_SUFFIX
  • Earlier MR: !84312 (merged)
• Change is to latest unstable template versions
• Specs updated
• No screenshot due to latest version
• No changelog needed, same milestone that _VERSION_TAG was introduced

Related to #354362 (closed)

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