Upgrade ion-java dependency to 1.12.1 #800

All maintained branches pin ion-java 1.11.11 (ion/pom.xml). That is the one release carrying a binary-write corruption bug, fixed in 1.12.0 by amazon-ion/ion-java#1150:

In ion-java 1.11.11, a bug in IonRawBinaryWriter causes corrupted Ion binary output when the writer is reused across multiple stream segments (via finish()). Stale PatchPoint objects from a previous write cycle are erroneously applied during the subsequent finish(), producing invalid Ion binary data.

Trigger conditions are binary writers, non-zero container length preallocation (1-byte is the default), plus either auto-flush enabled or an explicit IonWriter.flush() / finish(), followed by more values written to the same writer.

Jackson's own generator paths do not appear to trigger it: IonGenerator.flush() flushes the destination (OutputStream / Writer), not the IonWriter, and nothing calls finish(). Exposure is to callers who pass their own IonWriter to IonFactory.createGenerator(ObjectWriteContext, IonWriter) and flush or finish it themselves — but pinning the one release with a known binary-corruption bug is worth avoiding regardless.

1.12.0 / 1.12.1 also bring:

• IonCursorBinary grows its buffer by doubling as data is consumed, instead of allocating the full declared length up front (Grow reader buffer incrementally and add GZIP opt-out amazon-ion/ion-java#1153)
• text read hot path optimizations ([IonJava] Optimize text read hot paths (+18.1% aggregate JMH) amazon-ion/ion-java#1151)
• GZIP auto-decompression changes: new IonReaderBuilder.withGzipDecompressionEnabled opt-out (Grow reader buffer incrementally and add GZIP opt-out amazon-ion/ion-java#1153), and removal of a second layer of auto-decompression (Removes second layer of GZIP auto-decompression. amazon-ion/ion-java#1160)

1.12.1 remains Java 8 bytecode (major version: 52, OSGi JavaSE 1.8), so it is not a baseline change for any branch.

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