Update version to 20.0.0 and update CHANGELOG

[alamb]

Which issue does this PR close?

Re #2172

(Arrow 20.0.0 is looking pretty sweet 👌 )

Rationale for this change

Prepare for biweekly release

What changes are included in this PR?

1. Update version to 20.0.0
2. Update readme. Rendered preview: https://github.com/alamb/arrow-rs/blob/alamb/prepare_for_20.0.0/CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #2323 (87bb7c7) into master (299908e) will decrease coverage by 1.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
- Coverage   82.14%   80.53%   -1.62%     
==========================================
  Files         248      248              
  Lines       63492    59971    -3521     
==========================================
- Hits        52158    48296    -3862     
- Misses      11334    11675     +341     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/cast.rs	97.17% <100.00%> (+<0.01%)	⬆️
arrow/src/array/ffi.rs	0.00% <0.00%> (-100.00%)	⬇️
arrow/src/ffi.rs	0.00% <0.00%> (-87.53%)	⬇️
arrow/src/ffi_stream.rs	0.00% <0.00%> (-79.90%)	⬇️
arrow/src/datatypes/ffi.rs	0.00% <0.00%> (-76.66%)	⬇️
arrow/src/array/array_decimal.rs	88.60% <0.00%> (-2.10%)	⬇️
arrow/src/array/array_list.rs	94.27% <0.00%> (-1.57%)	⬇️
arrow/src/array/array_binary.rs	94.83% <0.00%> (-0.94%)	⬇️
arrow/src/array/array_string.rs	97.29% <0.00%> (-0.92%)	⬇️
parquet/src/util/bit_util.rs	93.14% <0.00%> (-0.79%)	⬇️
... and 18 more

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[ursabot]

Benchmark runs are scheduled for baseline = b1e2bd9 and contender = 9fde389. 9fde389 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java