MINOR: [1.18.x] Fix vector-plugin parent versions

[manuzhang]

Rationale for this change

Vector-plugins CI on parquet-1.18.x fails while reading the Maven reactor: both vector-plugin modules still reference parent version 1.18.0-SNAPSHOT, but the root POM is 1.18.2-SNAPSHOT. Maven rejects the local parent and cannot resolve the stale snapshot.

What changes are included in this PR?

Align the parent version in parquet-encoding-vector and parquet-plugins-benchmarks with the root POM's 1.18.2-SNAPSHOT. This PR is based directly on parquet-1.18.x and changes only these two version entries.

Are these changes tested?

• Reproduced the parent-resolution failure with the unmodified vector-plugin reactor.
• On JDK 17, ./mvnw -B -Pvector-plugins -pl parquet-plugins/parquet-encoding-vector,parquet-plugins/parquet-plugins-benchmarks -am validate spotless:check passed for all seven reactor modules.
• Confirmed both module parent versions match the root POM; git diff --check passed.
• Full reactor verify progressed past the original failure, then stopped in parquet-format-structures because the local Thrift compiler is unavailable. CI installs Thrift before building.

Are there any user-facing changes?

No. This repairs the maintenance branch's vector-plugin build configuration.