chore: upgrade go to 1.26

[dncrews]

Summary

Upgrade the backend Go toolchain to 1.26 and update related CI/tooling compatibility.

• Update Go versions in go.mod, Dockerfiles, devcontainer, and lake-builder image.
• Migrate golangci-lint to v2 and update the GitHub Action.
• Update mockery for Go 1.26 compatibility.
• Replace deprecated Go APIs flagged by the updated lint setup.
• Add local E2E setup notes, including the libgit2 rpath requirement.
• Update stale actions/* workflow versions.

Does this close any open issues?

No, but there are some related issues that are closed. See below.

Screenshots

N/A

Other Information

Follows up on the earlier Go upgrade work in #8599 by @petkostas.

Verified locally:

• make go-dep
• make mock
• make lint
• targeted Go tests for changed packages
• go run ./test/init.go
• E2E remote/services tests
• make unit-test-go
• workflow YAML parse check
• git diff --check

Suggested label: pr-type/feature-development or pr-type/maintenance