Remove branch from Debian package versions

[AlekSi]

To match Docker tags.

PR Overview

This PR simplifies the Debian package version naming by removing the branch indicator from the version string to align with Docker tags.

• Removed branch handling logic to only support the "ferretdb" branch.
• Updated Debian version formatting in the packaging code and corresponding expected outputs in tests.

Reviewed Changes

File	Description
ferretdb_packaging/defineversion/debian.go	Removed branch marker from version string and simplified branch handling logic.
ferretdb_packaging/defineversion/main_test.go	Adjusted expected Debian version strings in tests to match the updated version format.

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

ferretdb_packaging/defineversion/debian.go:98

• [nitpick] The function now only supports the 'ferretdb' branch; consider updating the function's comment to reflect this limitation for clarity.

if branch != "ferretdb" {

ferretdb_packaging/defineversion/debian.go:99

• Consider including the supported branch ('ferretdb') in the error message to provide clearer guidance for developers.

return "", fmt.Errorf("unhandled branch %q", branch)

ferretdb_packaging/defineversion/main_test.go:153

• Consider adding tests for scenarios when an unsupported branch is passed to ensure the error path is properly covered.

expectedDebian: "0.100.0~ferretdb",