Mark remove-old-releases.sh executable

[alamb]

Follow on to #1467

Rationale

When I ran the release-tarball.sh script for the 11.1.0 release, it said:

...
Success!
The release is available here:
  https://dist.apache.org/repos/dist/release/arrow/arrow-rs-11.1.0
Clean up old versions from svn
./dev/release/release-tarball.sh: line 79: /Users/alamb/Software/arrow-rs/dev/release/remove-old-releases.sh: Permission denied

[codecov-commenter]

Codecov Report

Merging #1522 (8c6b24f) into master (eb6b7c6) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 8c6b24f differs from pull request most recent head 3b3d9c8. Consider uploading reports for the commit 3b3d9c8 to get more accurate results

@@            Coverage Diff             @@
##           master    #1522      +/-   ##
==========================================
- Coverage   82.76%   82.75%   -0.01%     
==========================================
  Files         190      190              
  Lines       54702    54702              
==========================================
- Hits        45272    45270       -2     
- Misses       9430     9432       +2     

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.23%)	⬇️
arrow/src/array/transform/mod.rs	86.46% <0.00%> (-0.12%)	⬇️

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