chore: Update Go SDK to 1.26 and modernize codebase

[pweiber]

This PR aims to update the current go sdk to 1.26 and modernize some parts for the JWT sample

• Upgrade Go from 1.23 to 1.26 across go.mod, Dockerfile, CI workflow, and README
• Migrate JWT library from dgrijalva/jwt-go (unmaintained, CVE-prone) to golang-jwt/jwt/v5
• Harden JWT header extraction: case-insensitive comparison via strings.EqualFold and RawValue→Value fallback
• Apply Go modernizations: ioutil.ReadFile → os.ReadFile, interface{} → any, HasPrefix/TrimPrefix → CutPrefix, err == io.EOF → errors.Is(err, io.EOF)
• Replace deprecated grpc.Dial with grpc.NewClient
• Migrate github.com/golang/protobuf/proto to google.golang.org/protobuf/proto