RED-196948: Update process exporter to Go 1.26.3

[peperon]

Note

Low Risk
Toolchain-only version bumps with no runtime or security logic changes in the diff.

Overview
Bumps the process-exporter build toolchain from Go 1.25.8 to Go 1.26.3 everywhere the version is pinned.

go.mod declares go 1.26.3. GitHub Actions build and release workflows use that version via setup-go. The Dockerfile build stage uses golang:1.26.3, and Makefile dockertest / dockerinteg run inside the same image tag. No application source or exporter logic changes in this diff.

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[jit-ci]

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}