Update cudf ref main [skip ci]

[pxLi]

part of #581

This change must be merged before the new branching strategy pattern

[Copilot]

Pull Request Overview

This PR updates references from branch-25.12 to main to align with a new branching strategy. The changes ensure that development dependencies and documentation links point to the main branch instead of a specific release branch.

Key changes:

• Updated cudf dependency GIT_TAG from branch-25.12 to main in CMakeLists.txt
• Updated Google Colab notebook URL from branch-25.12 to main in README documentation

Reviewed Changes

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

File	Description
examples/UDF-Examples/RAPIDS-accelerated-UDFs/src/main/cpp/CMakeLists.txt	Updates cudf dependency to track main branch
examples/SQL+DF-Examples/tpcds/README.md	Updates Colab notebook link to reference main branch

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