Fix docs/_code/samples build on Apple Silicon

[alexander-yevsyukov]

What

Makes the docs/_code/samples Gradle build succeed again. Verified with
./gradlew -p docs/_code/samples clean buildAll (compileJava, compileTestJava,
and test all pass) on an arm64 Mac.

Why

spine.enableJava().server() (Spine Bootstrap 1.7.0, from 2020) drives all
repository and codegen-tool configuration, and two things it sets are now broken:

1. Dead repository. Bootstrap injects the shut-down JCenter as the source
   for third-party artifacts such as protoc. Nothing resolves from it.
2. No Apple Silicon binaries. It pins protoc:3.13.0 and
   protoc-gen-grpc-java:1.28.1, which predate Apple Silicon — no osx-aarch_64
   build of either exists at any repository, so :generateProto fails on arm64 Macs.

What changed

Only docs/_code/samples/build.gradle, after spine.enableJava().server():

• Remove the dead BintrayJCenter repo and add mavenCentral(). Spine's own
  artifacts keep resolving from the spine.mycloudrepo.io repositories Bootstrap
  also injects — only the third-party source is repointed.
• On arm64 macOS only, force the osx-x86_64 protoc and gRPC binaries
  (...:osx-x86_64@exe). They run under Rosetta 2 and stay pinned at the exact
  Bootstrap-selected versions, so generated code is identical to every other
  platform. Non-mac platforms keep the auto-detected native binary, so the Linux
  CI job is unaffected. Versions are unchanged — the v1 Bootstrap/protoc line is
  preserved.

Reviewer notes

• CI already exercises this. check-code-embedding.yml → build-embedded-code
  runs ./gradlew :buildAll (root task fans out to every included build) on
  ubuntu-latest/JDK 8. It's Linux-x86_64 only, which is why this only ever broke
  local arm64 Mac builds. This change also moves samples off the sunset JCenter,
  hardening that job.
• Sibling examples have the same arm64 gap (examples/hello, Bootstrap 1.9.0;
  examples/airport, Bootstrap 1.8.0) — both fail on protoc-3.13.0-osx-aarch_64.
  Those are pinned spine-examples/* submodules, so the equivalent fix belongs
  upstream and is not included here.
• gradlew.bat shows as modified in the working tree — an unrelated pre-existing
  line-ending change, deliberately left out of this PR.

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