[physical-plan]: remove deprecated UnionExec::new

[megakaizo]

Which issue does this PR close?

• part of Remove old deprecated code #23080

Rationale for this change

UnionExec::new is deprecated in 44.0.0, and replaced UnionExec::try_new

What changes are included in this PR?

Removed the deprecated UnionExec::new constructor

Are these changes tested?

verified by running local tests for datafusion-physical-plan package

Are there any user-facing changes?

Yes. This removes a public Rust API UnionExec::new that was deprecated in 44.0.0. Downstream users should migrate to UnionExec::try_new.

This is an API change and should be labeled api change.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-physical-plan v54.0.0 (current)
       Built [  29.189s] (current)
     Parsing datafusion-physical-plan v54.0.0 (current)
      Parsed [   0.113s] (current)
    Building datafusion-physical-plan v54.0.0 (baseline)
       Built [  28.297s] (baseline)
     Parsing datafusion-physical-plan v54.0.0 (baseline)
      Parsed [   0.111s] (baseline)
    Checking datafusion-physical-plan v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.689s] 223 checks: 222 pass, 1 fail, 0 warn, 30 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/inherent_method_missing.ron

Failed in:
  UnionExec::new, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/75d3c87db2328833532578616ff1c9c11e735e05/datafusion/physical-plan/src/union.rs:116

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  59.806s] datafusion-physical-plan

[alamb]

Thank you @megakaizo