chore: update lance dependency to v6.0.0-rc.2 (#496)

## Summary
- Bump `lance.version` to `6.0.0-rc.2`.
- Update Docker `LANCE_SPARK_VERSION` to the current
`lance-spark.version` (`0.4.0-beta.4`).
- Confirm `LANCE_NS_VERSION` remains `0.7.2`, matching the
`lance-namespace-core` dependency in the tagged `lance-core` POM.

## Verification
- `make lint` passed.
- `make install` passed for the default Spark 3.5 / Scala 2.12 build
after installing `org.lance:lance-core:6.0.0-rc.2` from the upstream tag
into the local Maven cache; Maven Central metadata did not yet list
`6.0.0-rc.2` during this run.

Triggering tag: refs/tags/v6.0.0-rc.2

Author: lance-community

docker/Dockerfile

@@ -42,7 +42,7 @@ WORKDIR ${SPARK_HOME}
 ENV SPARK_VERSION=${SPARK_DOWNLOAD_VERSION}
 ENV SPARK_MAJOR_VERSION=${SPARK_MAJOR_VERSION}
 ENV SCALA_VERSION=${SCALA_VERSION}
-ENV LANCE_SPARK_VERSION=0.4.0-beta.3
+ENV LANCE_SPARK_VERSION=0.4.0-beta.4
 ENV LANCE_NS_VERSION=0.7.2
 
 # Download spark

pom.xml

@@ -51,7 +51,7 @@
 
     <properties>
         <lance-spark.version>0.4.0-beta.4</lance-spark.version>
-        <lance.version>6.0.0-beta.4</lance.version>
+        <lance.version>6.0.0-rc.2</lance.version>
         <lance-namespace.version>0.7.2</lance-namespace.version>
         <lance-namespace-impl.version>0.3.0</lance-namespace-impl.version>