fix: opcm2 upgradeopchain comments

[0xiamflux]

Changes:

• Package version bump: Renamed v6_0_0 package to v7_0_0 (upgrade.go, upgrade_test.go, testdata/config.json)
• Remove Cannon Kona feature flags: Removed Cannon Kona DGV2 dev feature flags from integration tests
• Link TODO to issue: Added issue reference Add Resource Locator for v7_0_0 artifacts ethereum-optimism/optimism#18633 to TODO comment in ArtifactsURL() method
• Remove unused test function: Deleted mustEncodeGameArgs helper function from integration tests
• Improve config validation: Changed input.ChainConfigs == nil to len(input.ChainConfigs) == 0 for proper nil and empty slice handling

---

Note

Renames the upgrader package to v7_0_0, improves upgrade input validation, removes Cannon Kona/DGV2 test variants, and deletes an unused test helper.

• Upgrade package:
  • Rename package from v6_0_0 to v7_0_0 in upgrade.go and upgrade_test.go.
  • Improve input validation in Upgrade(...): use len(input.ChainConfigs) == 0 to detect empty configs.
  • Update TODO in ArtifactsURL() to reference issue #18633.
• Integration tests (integration_test/apply_test.go):
  • Remove deploy-v2-disputegames and cannon-kona test variants; keep default and opcm-v2.
  • Delete unused mustEncodeGameArgs helper.

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