Core: Remove deprecated DataReader in favor of PlannedDataReader

[dramaticlly]

Removes Avro read paths deprecated for removal in 1.12.0:

• org.apache.iceberg.data.avro.DataReader (class) → PlannedDataReader
• RawDecoder(Schema, Function<Schema, DatumReader<?>>, Schema) → RawDecoder.create(Schema, Function, Schema)

Neither has any remaining reference in main or test sources. TestDataReader is removed along with the class it covered; PlannedDataReader has its own coverage.

Split out of #16449 to reduce reviewer burden.

AI Disclosure

Model: Claude Opus 5 (1M context)
Platform/Tool: Claude Code
Human Oversight: reviewed
Prompt Summary: split #16449 into smaller self-contained PRs; verify each group compiles and tests green standalone

[dramaticlly]

@gaborkaszab can you help check for removal match the expectation of #14387

[uros-b]

+1!

[dramaticlly]

I rebased latest upstream main for resolving conflict in revapi.yml, but looks like spark CI runs into throttling problem, might need maintainer to kick off another run.

from https://github.com/apache/iceberg/actions/runs/32197106613/job/95903257724?pr=17699

* What went wrong:
Execution failed for task ':iceberg-spark:iceberg-spark-4.0_2.13:test'.
You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.
> Could not resolve all files for configuration ':iceberg-spark:iceberg-spark-4.0_2.13:testRuntimeClasspath'.

   > Could not resolve com.microsoft.sqlserver:mssql-jdbc:6.2.1.jre7.
For more on this, please refer to https://docs.gradle.org/8.14.5/userguide/command_line_interface.html#sec:command_line_warnings in the Gradle documentation.
     Required by:
         project :iceberg-spark:iceberg-spark-4.0_2.13 > org.apache.hadoop:hadoop-minicluster:3.4.3 > org.apache.hadoop:hadoop-yarn-server-tests:3.4.3 > org.apache.hadoop:hadoop-yarn-server-common:3.4.3
      > Could not resolve com.microsoft.sqlserver:mssql-jdbc:6.2.1.jre7.
         > Could not get resource 'https://repo.maven.apache.org/maven2/com/microsoft/sqlserver/mssql-jdbc/6.2.1.jre7/mssql-jdbc-6.2.1.jre7.pom'.
            > Could not GET 'https://repo.maven.apache.org/maven2/com/microsoft/sqlserver/mssql-jdbc/6.2.1.jre7/mssql-jdbc-6.2.1.jre7.pom'. Received status code 429 from server: Too Many Requests

[huaxingao]

@dramaticlly could you resolve the conflicts?

[huaxingao]

Thanks @dramaticlly for the PR! Thanks everyone for the review!