Update k6 from v1.5.0 to v2.0.0

[moko-poi]

Summary

• Bump k6 from v1.5.0 to v2.0.0 (new major stable release) in k6.json, the single source of truth read at runtime by deployment/ecr/distributed-load-testing-on-aws-load-tester/load-test.sh.
• The download URL templates in k6.json are unchanged: the v2.0.0 release publishes the same asset naming the templates already expect.

Verification

• Confirmed the resolved download URLs return HTTP 200:
  • k6-v2.0.0-linux-amd64.tar.gz
  • k6-v2.0.0-linux-arm64.tar.gz
  • k6-v2.0.0-checksums.txt
• Confirmed the checksums file lists both linux tarballs under the exact archive names load-test.sh greps for, so the download + checksum-validation + extraction flow works unchanged.
• Repo-wide search confirms the solution does not directly use any feature removed in k6 v2.0 (externally-controlled executor, --no-summary, --summary-mode=legacy, k6 login, options.ext.loadimpact, k6/experimental/redis, K6_BINARY_PROVISIONING, K6_ENABLE_COMMUNITY_EXTENSIONS).

Notes

• CHANGELOG is intentionally left unchanged; following the existing convention, the k6 upgrade entry is added at release time (the v1.5.0 bump was recorded under [4.0.7]).
• k6 is invoked indirectly via Taurus (bzt). End-to-end validation of the bundled Taurus k6 executor against v2.0 is recommended before release — see issue Update k6 from v1.5.0 to v2.0.0 #314 for the compatibility checklist.

Test plan

• Build the load-tester container and run a k6 .js scenario (single file) end-to-end; confirm kpi.csv and results.xml are produced and parsed.
• Run a k6 .ts scenario and a zipped scenario.
• Verify on both amd64 and arm64 task architectures.

Closes #314