chore(publish): cut Maven Central file and release counts

[rossdanderson]

Maven Central's Central Portal enforces per-organisation monthly publishing limits (file count, release size, release count) as three-month rolling averages, with hard rate-limiting from 2026-08-11. We were publishing ~380 files per release and counting every RC tag as a full release, putting us over the file-count and release-count thresholds. Two independent levers:

File count — bump vanniktech-maven-publish 0.31.0 → 0.37.0. Its defaults publish only md5/sha1 checksums and drop the redundant checksums-of-.asc-signatures and sha256/sha512 (none of which Central reads). Cuts each artifact from 10 published files to 4 — ~380 → ~152 files per release. Also fixes the removed SonatypeHost API (publishToMavenCentral() no-arg).

Release count — derive the publish version from the git tag: a pre-release tag (X.Y.Z-rcN, case-insensitive) publishes an overwriting X.Y.Z-SNAPSHOT to the separate, quota-exempt snapshot repository; a clean X.Y.Z tag still releases. Turns a version that previously cost 3–4 counted releases into 1.

Notes

• No public .api change; no CI change (publish job still fires on every tag — RC tags now publish a snapshot live, final tags stage a release for the usual manual "Publish").
• 0.37.0's prerequisites (Gradle 9.5.1, Kotlin 2.2.21, Dokka v2 2.2.0) were already met, so the only breaking change that applied was the SonatypeHost rename.
• Requires SNAPSHOTs enabled for the com.caplin.integration.datasourcex namespace (already done).
• Follow-up: port the same two edits to springboot-3.5.x (identical toolchain, clean cherry-pick).

Verification

• ./gradlew classes — BUILD SUCCESSFUL (fixes the current SonatypeHost CI compile failure)
• spotlessApply clean
• Version mapping confirmed: 3.1.0-rc2/3.1.0-RC1 → 3.1.0-SNAPSHOT, 3.1.0 → 3.1.0, dev → dev