Deprecate the Calculation API Spark cursors #1305

Part of #1301. The design is agreed in #1302.

Problem

SparkCursor, AsyncSparkCursor, and AioSparkCursor only work on PySpark engine version 3 workgroups, which stop running sessions and calculations on 2027-04-01.
Users get no warning before then.

Proposed change

The timing and wording follow #1302.

• Emit a DeprecationWarning from the three cursors that names the end-of-life date and points to the Apache Spark version 3.5 API.
• Note the end of life and the migration path in docs/spark.md.