GH-48735: [Python] Fix pyproject.toml license format for PEP 621 compliance

[HyukjinKwon]

Rationale for this change

Python wheel builds are failing with:

ValueError: invalid pyproject.toml config: `project.license`.
configuration error: `project.license` must be valid exactly by one definition (2 matches found)

This was introduced in commit 866502e which changed the license to:

license = "Apache-2.0"

However, PEP 621 requires the license field to be a table with either text or file key, not a plain string.

What changes are included in this PR?

Changed line 35 from:

license = "Apache-2.0"

to:

license = {text = "Apache-2.0"}

(I believe this can also be license = {file = "LICENSE.txt"} but kept the original intention from the commit).

This maintains SPDX compliance while fixing PEP 621 compliance.

Are these changes tested?

Should trigger the build to make sure it passes all fine with CI

Are there any user-facing changes?

No, dev-only.

• GitHub Issue: [CI][Python] macos wheels are failing to build due to project.license error #48735

[github-actions]

⚠️ GitHub issue #48735 has been automatically assigned in GitHub to PR creator.