Prepare for release 21.0.0 -- update version and CHANGELOG.md

[alamb]

Which issue does this PR close?

RE #2382

Rationale for this change

What changes are included in this PR?

1. Update versions of parquet, parquet-derive, arrow and arrow-flight to 21.0.0
2. Update changelog. Rendered version https://github.com/alamb/arrow-rs/blob/alamb/prepare_for_21.0.0/CHANGELOG.md

Are there any user-facing changes?

Yes

[ursabot]

Benchmark runs are scheduled for baseline = 526a2b8 and contender = dacd354. dacd354 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