build: bump R8 to 9.1.31 for Kotlin 2.3 metadata parsing

[OmarAlJarrah]

Problem

The test-only sdk-shrink-test module pinned R8 8.9.35. That R8 line's
Kotlin-metadata support only covers Kotlin up to 2.1, but this project
compiles with Kotlin 2.3. As a result every R8 shrink run emitted non-fatal
Info/Warning lines reporting that it could not parse the Kotlin 2.3
@Metadata annotations.

The shrunk output stayed correct — @Metadata is retained via the shipped
-keepattributes RuntimeVisibleAnnotations / -keep class kotlin.Metadata
rules, and the consumer program's Jackson kotlin-reflect round-trip passes —
but the per-run noise risked masking a genuinely relevant warning.

Change

Bump the r8 version in gradle/libs.versions.toml from 8.9.35 to
9.1.31, the current stable R8 release, which parses Kotlin 2.3 metadata.

R8 9.x is still Java-11 bytecode, so the JDK-11 launcher and --lib boot
image in sdk-shrink-test are unchanged; no build-script edits were needed.

Verification

JDK 11 (Microsoft OpenJDK 11.0.28) and network access to Google's Maven repo
were both available, so the shrink pipeline was exercised end to end:

• ./gradlew :sdk-shrink-test:r8Run --no-daemon — BUILD SUCCESSFUL; the
  shrunk consumer printed the SHRINK-SURVIVAL-OK sentinel.
• ./gradlew :sdk-shrink-test:r8Shrink --rerun-tasks --no-daemon then
  grepping the output for metadata / cannot parse / Kotlin Info/Warning
  lines — none emitted by R8. The shrink run is now quiet, confirming the
  bump resolves the metadata-parse noise.

Closes #112

[OmarAlJarrah]

Straightforward dependency bump: R8 goes from 8.9.35 to 9.1.31 in gradle/libs.versions.toml, with a comment explaining that the older 8.9.x metadata parser topped out at Kotlin 2.1 and emitted noise now that we compile with 2.3.21. Looks good to merge.

The change is scoped to the single version-catalog entry and only affects the test-only sdk-shrink-test shrink-survival guard; no source, public API, or build logic is touched. The version is a real published R8 release on Google's Maven repo, the catalog edit is well-formed, and the added comment documents the motivation for the next person who looks at this line.