MINOR: rename optimizer rule to ScalarSubqueryToJoin

[kmitchener]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

Just renames optimizer rule DecorrelateScalarSubquery to ScalarSubqueryToJoin to more accurately reflect what it's doing. Per request of @avantgardnerio during discussion of #3287.

Are there any user-facing changes?

No

[jackwener]

It's reasonable to me.

[codecov-commenter]

Codecov Report

Merging #3306 (990b191) into master (fcd2275) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3306   +/-   ##
=======================================
  Coverage   85.74%   85.75%           
=======================================
  Files         294      294           
  Lines       53738    53749   +11     
=======================================
+ Hits        46080    46092   +12     
+ Misses       7658     7657    -1     

Impacted Files	Coverage Δ
datafusion/core/src/execution/context.rs	78.24% <100.00%> (ø)
...atafusion/optimizer/src/scalar_subquery_to_join.rs	93.07% <100.00%> (ø)
datafusion/optimizer/tests/integration-test.rs	84.00% <100.00%> (ø)
datafusion/expr/src/window_frame.rs	92.43% <0.00%> (-0.85%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	78.73% <0.00%> (ø)
datafusion/common/src/scalar.rs	84.71% <0.00%> (+0.06%)	⬆️
datafusion/proto/src/to_proto.rs	51.07% <0.00%> (+0.08%)	⬆️
datafusion/proto/src/lib.rs	93.52% <0.00%> (+0.11%)	⬆️
datafusion/proto/src/from_proto.rs	35.13% <0.00%> (+0.11%)	⬆️

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[alamb]

Thanks @kmitchener

[ursabot]

Benchmark runs are scheduled for baseline = 312a7dd and contender = 516ad0d. 516ad0d 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