Update version to 31.0.0 and add changelog

[iajoiner]

Which issue does this PR close?

Re #3415.

Rationale for this change

Biweekly release

What changes are included in this PR?

Update version to 31.0.0 and add changelog

See rendered changelog here: https://github.com/spaceandtimelabs/arrow-rs/blob/preparation_for_31.0.0/CHANGELOG.md

Are there any user-facing changes?

Update version to 31.0.0 and changelog

[alamb]

Hi @iajoiner -- I don't see any changelog in this PR (which is fine, but the title suggests it would be present so I am not sure of your intent)

[iajoiner]

@alamb Sorry I met with some secondary restrictions earlier today. Back to that now.

[iajoiner]

@alamb Now everything is correct. Sorry for the confusion.

[ursabot]

Benchmark runs are scheduled for baseline = 79d823f and contender = 8688dba. 8688dba 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