Avoid hardcoding `kong-effective-version` in targeted E2E workflow

[tao12345666333]

Is there an existing issue for this?

• I have searched the existing issues

Does this enhancement require public documentation?

• I have added an Acceptance Criteria item for adding and/or adjusting public documentation (if applicable)

Problem Statement

The targeted E2E workflow currently hardcodes kong-effective-version to 3.15.0 for the kong-gateway-dev:nightly image.

This is acceptable as a short-term workaround, but keeping this value hardcoded long term can make the workflow stale when the nightly Kong Gateway version changes. We should track this so the version is either derived automatically or maintained in a less fragile way.

ref： #8043 (comment)

Proposed Solution

Replace the hardcoded kong-effective-version in .github/workflows/e2e_targeted.yaml with a more maintainable approach, such as:

• Maybe we can get the latest version number of Kong from an interface on developer.konghq.com and set it as the effective-version?
• deriving the effective Kong Gateway version from the tested image (I'm not sure, but I can try to talk with gateway team)

Additional information

No response

Acceptance Criteria

• .github/workflows/e2e_targeted.yaml no longer relies on a stale-prone hardcoded kong-effective-version for the nightly Kong Gateway image, or the hardcoded value is centralized with a documented update path.

• Targeted E2E tests still receive the correct effective Kong Gateway version when testing kong/kong-gateway-dev:nightly.