[Dependencies] Bump versions

[weilerN]

📝 Description

Bumps the toolchain, base images, and indirect dependencies for the scaler. Brings Go to 1.26, the build/runtime images up to date, and upgrades golangci-lint (v2.7.2 → 2.12.1) with a more robust install path that pins and verifies the linter version.

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🛠️ Changes Made

• Bump Go 1.25.5 → 1.26.3 (go.mod).
• Bump base images: golang:1.25-alpine → 1.26-alpine and alpine:3.18 → 3.23 in cmd/autoscaler/Dockerfile and cmd/dlx/Dockerfile; golang:1.25 → 1.26 in test/Dockerfile.
• Bump golangci-lint v2.7.2 → 2.12.1 and refactor the Makefile: new ensure-golangci-linter target that installs the pinned version and reinstalls on a version mismatch; lint/fmt now depend on it.
• Add a .golangci.yml exclusion to skip goconst on _test.go files (test fixtures commonly repeat literals).
• Bump indirect deps in go.mod/go.sum: golang.org/x/net, x/sys, x/term, x/text, x/tools.

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✅ Checklist

• I have tested the changes in this PR

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🧪 Testing

• Ran make fmt lint with the new pinned golangci-lint version.
• Built the autoscaler, dlx, and test images against the bumped base images.

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🔗 References

• Ticket link: https://ecliptos.atlassian.net/browse/IG4-2332
• Design docs links:
• External links:

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🚨 Breaking Changes?

• Yes (explain below)
• No

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🔍️ Additional Notes

• Dependency/version bump only — no functional changes to autoscaler or dlx behavior.
• ensure-golangci-linter was taken from orca, same behavior.