Manual changelog for 20.0.0

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Trying to get the release candidate created.

I cannot use the changelog generator due to #5549

This is far from ideal but all I have time to do today. We can either create the release candidate with this changelog, or postpone te release until we have time to do something better.

What changes are included in this PR?

Manual changelog

• Updated changelog (rendered)

Are these changes tested?

Are there any user-facing changes?

[alamb]

I think this is good enough for today. I agree it could be better but 👍 thank you @andygrove

[ursabot]

Benchmark runs are scheduled for baseline = a768004 and contender = 860918d. 860918d 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-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-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