arrow

GH-44706: [Release][Archery][Packaging] Add "so_version" variable (#44707)

### Rationale for this change

SO versions must be computed from the target version instead of embedding them to `dev/tasks/tasks.yaml`. Because they depend on the specified version as an argument.

### What changes are included in this PR?

Add `so_version` variable that can be used in `dev/tasks/tasks.yml`.

### Are these changes tested?

Yes.

https://github.com/apache/arrow/pull/44699#issuecomment-2472056561 used this.

### Are there any user-facing changes?

No.
* GitHub Issue: #44706

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>