chore: bump golang to 1.26.4 for cve fixes

[lilypan26]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

[Copilot]

Pull request overview

This PR bumps the declared Go toolchain version to Go 1.26.4 across the repository’s Go modules (root module plus several submodules), aligning the codebase with a newer Go release intended to pick up CVE fixes.

Changes:

• Update go directives from 1.25.10 to 1.26.4 in the root module and multiple submodules.
• Keep dependency sets unchanged (no require/replace edits shown), focusing purely on the minimum Go version.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
go.mod	Bump root module minimum Go version to 1.26.4
aks-node-controller/go.mod	Bump aks-node-controller module minimum Go version to 1.26.4
image-fetcher/go.mod	Bump image-fetcher module minimum Go version to 1.26.4
vhdbuilder/lister/go.mod	Bump vhdbuilder lister module minimum Go version to 1.26.4
vhdbuilder/prefetch/go.mod	Bump vhdbuilder prefetch module minimum Go version to 1.26.4
hack/tools/go.mod	Bump hack/tools module minimum Go version to 1.26.4

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

[aks-node-assistant]

AgentBaker Linux gate detective

Run: https://msazure.visualstudio.com/CloudNativeCompute/_build/results?buildId=169093999
Failed job/stage/task: �2e / Run AgentBaker E2E
Summary: 210 E2E scenarios failed quickly during cluster preparation with Operation cannot be fulfilled on daemonsets.apps "e2e-proxy": the object has been modified, followed by VMSS paging cancellation. Failures span OS/SKU families before VM validation, so this points to shared E2E harness/cluster prep rather than VHD contents.

Likely cause/signature: shared �2e-proxy DaemonSet optimistic-concurrency race (�2e-cluster-prep-e2e-proxy-daemonset-conflict). Confidence: High.
Alternative considered: Go/toolchain PR broke E2E binaries; less likely because the repeated error is a Kubernetes object-version conflict in shared setup.
Recommended owner/action: E2E harness owner: retry-on-conflict around �2e-proxy DaemonSet create/update or serialize shared proxy setup; rerun PR gate.
Wiki: $(https://dev.azure.com/msazure/09706533-03bf-4b43-9a9b-b49c75429646/_wiki/wikis/ed4a85e9-1085-4151-a39b-2753523eba2b?pagePath=%2FAKS%2FSIGs%20and%20Teams%2FAKS%20Components%2FSIG%3A%20Node%20Lifecycle%2FAI%20Agent%20Knowledge%2FAgentBaker%20Gate%20PR%20Pipeline%20Flakiness) (�2e-cluster-prep-e2e-proxy-daemonset-conflict)