Bump GAIE dependency to v1.4.0

[danehans]

Summary

Dynamo's GAIE integration currently pins sigs.k8s.io/gateway-api-inference-extension to v1.2.1, and the install/docs flow for GAIE uses the same v1.2.1 release.

That is appropriate for the current #6331 agentgateway migration PR, but it leaves Dynamo behind the current upstream GAIE release line.

Current State

• EPP dependency pin: deploy/inference-gateway/epp/go.mod -> v1.2.1
• GAIE install/uninstall docs and scripts also use v1.2.1
• Upstream GAIE latest release is v1.4.0

Follow-up Work

• Bump the Dynamo EPP GAIE dependency from v1.2.1 to v1.4.0
• Update install/uninstall scripts and docs to use v1.4.0
• Review any API or behavior changes between v1.2.1 and v1.4.0
• Validate Dynamo + GAIE e2e after the bump

Notes

This is being handled separately from #6331, which is focused on migrating the gateway implementation from kgateway to agentgateway while preserving the current working GAIE dependency baseline.

[danehans]

/assign