codewide: rename remnants of cluster data to state

[wprzytula]

There were multiple places where ClusterState was still referred to as ClusterData:

• Session::get_cluster_data(),
• Cluster::get_data(),
• variable names,
• comments and docstrings.

All were renamed.

Supplements #1163.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• [ ] I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: 6aaee31

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 448502ca1db6228ab3d1d6108db78664e1dfddb6
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 448502ca1db6228ab3d1d6108db78664e1dfddb6
     Cloning 448502ca1db6228ab3d1d6108db78664e1dfddb6
    Building scylla v0.15.0 (current)
       Built [  23.019s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.050s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  22.728s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.048s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.131s] 127 checks: 126 pass, 1 fail, 0 warn, 0 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/inherent_method_missing.ron

Failed in:
  GenericSession::get_cluster_data, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-448502ca1db6228ab3d1d6108db78664e1dfddb6/65b2eff398a4ccb7c9c581eed694c6b5b77c88c7/scylla/src/client/session.rs:1737

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  47.200s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  11.503s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.034s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.888s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.033s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.129s] 127 checks: 127 pass, 0 skip
     Summary no semver update required
    Finished [  23.212s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[Lorak-mmk]

@roydahan Is there a reason you assigned me? I'm not working on this PR, I just have to review it (and I was already in reviewers list).