Update Go version to 1.26 and adjust workflows and Dockerfile

[marcschaeferger]

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Description

This pull request updates the project's Go version to 1.26 and improves version management by centralizing the Go version in the .go-version file. The workflows, Dockerfile, and Makefile are all updated to consistently use this version, reducing the risk of mismatched Go versions across development, CI, and production builds.

Go version update and centralization:

• Updated the Go version from 1.25 to 1.26 in the .go-version file, making 1.26 the new project standard.
• Modified the GitHub Actions workflow (.github/workflows/test.yml) to read the Go version from .go-version and set up the workflow environment accordingly, ensuring CI always uses the correct version.
• Updated the Dockerfile to accept a GO_VERSION build argument and default to the version from .go-version, ensuring Docker builds use the intended Go version.

Build process improvements:

• Added logic to the Makefile to read the Go version from .go-version, and updated the docker-build-release and build targets to pass the correct version to Docker builds. [1] [2]

How to test?