Update Spark compatibility and installation guidance

[jkasireddy]

Issue #, if available:

N/A — documentation update.

Description of changes:

• Updated the README to clarify that Deequ 2.x artifacts are built for specific Apache Spark 3.x versions.
• Added a Spark/Deequ compatibility table covering Spark 3.1.x through Spark 3.5.x.
• Updated the Maven and SBT dependency examples to use 2.0.21-spark-3.5.
• Removed ambiguous wording from the existing compatibility guidance.
• This is a documentation-only change and does not modify application behavior.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

No issues found.

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