Arrow 20.0.0 official release candidiate also has `object_store` crate code in it

[alamb]

Describe the bug

Now that we have added the object_store crate to the workspace #2081 it is included by default in our source builds

While we don't plan to release this object_store code to crates.io it may be confusing to have an "approved" release that has it there

To Reproduce
Look at release candidate: https://dist.apache.org/repos/dist/dev/arrow/apache-arrow-rs-20.0.0-rc1/apache-arrow-rs-20.0.0.tar.gz

You will see object store in it:

$ wget https://dist.apache.org/repos/dist/dev/arrow/apache-arrow-rs-20.0.0-rc1/apache-arrow-rs-20.0.0.tar.gz

Now look at the contents:

$ tar tf apache-arrow-rs-20.0.0.tar.gz  | grep object_store | head
apache-arrow-rs-20.0.0/.github/workflows/object_store.yml
apache-arrow-rs-20.0.0/object_store/
apache-arrow-rs-20.0.0/object_store/CONTRIBUTING.md
apache-arrow-rs-20.0.0/object_store/Cargo.toml
apache-arrow-rs-20.0.0/object_store/README.md
apache-arrow-rs-20.0.0/object_store/deny.toml
apache-arrow-rs-20.0.0/object_store/src/
apache-arrow-rs-20.0.0/object_store/src/aws.rs
apache-arrow-rs-20.0.0/object_store/src/azure.rs
apache-arrow-rs-20.0.0/object_store/src/client/

Expected behavior
object_store should not be in the arrow release tarball (we hope to have its own release cadence)

I started hacking on the release scripts during the 20.0.0 release to remove it from the tarball, but that caused at least two issues:

1. The Cargo.toml in the top level directory still refers to object_store so cargo test gives an error about "workspace member not found"
2. If I also deleted Cargo.toml then the verification script won't work as written (as it calls cargo test in the workspace root)

I suppose I could also update the verification script to test the crates individually, e.g.

cargo test -p arrow
cargo test -p arrow-flight
cargo test -p parquet
cargo test -p parquet_derive

Additional context
Found while working to release arrow 20, #2172

[alamb]

Last call -- unless I hear differently I plan to remove object_store code in the 21.0.0 release