Prepare release 2.1.2

[emmanuel-keller]

Release 2.1.2. Sets the release version in build.gradle and the README stable-install example, dates the CHANGELOG section, and backfills the changelog entries for #183 and #184 (both landed on main without one).

Included since 2.1.1

• Add full geometry-type support (read + write) #183 — Full geometry-type support (read + write): all seven types, type-discrimination accessors, Point2D.Double readers, and write factories. Also fixes an UnsatisfiedLinkError on Geometry finalization.
• Add live query kill() and expose the live-query UUID up front #184 — Live-query kill() (Surreal.kill(String) / kill(UUID)) and LiveStream.getQueryId() exposing the live-query UUID up front.
• Bump SurrealDB to 3.1.5, run CI against server v3.1.5 #185 — Upgrade to SurrealDB SDK 3.1.5; CI runs against server v3.1.5.

Release mechanics

• Pushing this non-SNAPSHOT version to main does not auto-publish (the publish job skips non-SNAPSHOT pushes); the release is published via manual workflow_dispatch after merge.
• Follow-up PR will bump main to 2.1.3-SNAPSHOT.