JGC-486 - Bump Go toolchain to 1.26.3

[RemiBou]

Summary

Aligns the JFrog CLI repository with the latest stable Go release (1.26.3, 2026-05-07 per Go release history).

Changes

• go.mod: go directive 1.25.7 → 1.26.3
• Jenkinsfile: Go tool go-1.25.7 → go-1.26.3 (ensure this tool is installed on Jenkins agents)
• prepareDarwinBinariesForRelease workflow: go-version 1.26.3
• build/docker/slim/Dockerfile: builder image golang:1.26.3-alpine (confirm image exists in the internal registry)

go mod tidy was run with Go 1.26.3 (minor go.sum cleanup).

Tracking

JGC-486

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