docs: Fix installation instructions for golangci-lint v2 compatibility

[krishagarwal278]

@JoelSpeed I've updated the README to fix the installation issues.

Main changes:

• Module mode now uses v2.5.0 and includes the pseudo-version format you showed in the cluster-api example, plus instructions on how to generate it
• Fixed the plugin build instructions to use the vendor directory like you suggested:

  go mod vendor
  go build -mod=vendor -buildmode=plugin -o bin/kube-api-linter.so ./vendor/sigs.k8s.io/kube-api-linter

• Added version: "2" to all the .golangci.yml examples
• Fixed up the Makefile and standalone build docs to be consistent with the actual directory structure

[k8s-ci-robot]

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[krishagarwal278]

I've tested the standalone binary build with make build and it works fine. Let me know if anything needs adjusting!