Prepare for arrow 15.0.0 release

[alamb]

Which issue does this PR close?

RE #1727

Rationale for this change

Release arrow 15.0.0

What changes are included in this PR?

1. Update versions
2. Add 15.0.0 change log. Rendered version https://github.com/alamb/arrow-rs/blob/alamb/prepare_for_15.0.0/CHANGELOG.md
3. Update scripts to help me generate changelog

Are there any user-facing changes?

Changelog

[codecov-commenter]

Codecov Report

Merging #1751 (8183557) into master (8e1666a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1751   +/-   ##
=======================================
  Coverage   83.47%   83.48%           
=======================================
  Files         196      196           
  Lines       55923    55923           
=======================================
+ Hits        46684    46685    +1     
+ Misses       9239     9238    -1     

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
arrow/src/array/transform/mod.rs	86.85% <0.00%> (+0.11%)	⬆️
parquet/src/encodings/encoding.rs	93.65% <0.00%> (+0.19%)	⬆️

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