docs: use version placeholders across dependency snippets

[limehee]

Summary

This PR updates documentation dependency snippets to reduce version maintenance overhead and improve copy-paste safety.

Changes included:

• Replaced hardcoded dependency versions in docs with placeholders/variables.
• Standardized snippet style across Maven, Gradle Kotlin DSL, and Gradle Groovy DSL.
• Added Version Catalog (libs.versions.toml) examples for both starter and core artifacts.
• Added explicit guidance to replace latest-version with an actual release version.
• Generalized issue template version placeholder to avoid stale fixed examples:
  • .github/ISSUE_TEMPLATE/bug_report.yml

Updated files:

• README.md
• docs/getting-started.md
• docs/spring-boot-guide.md
• docs/pure-java-guide.md
• .github/ISSUE_TEMPLATE/bug_report.yml

Related Issues

• Closes Use version placeholders in README/docs to reduce manual version maintenance #12
• Related #

Change Type

• Bug fix
• Feature
• Refactor
• Documentation
• Test
• Build/CI

JSON-RPC Impact

Describe protocol-level impact, if any:

• No protocol behavior change
• Request validation behavior changed
• Error mapping behavior changed
• Method registration/dispatch behavior changed

Details:

• Documentation-only updates.
• No runtime behavior or protocol semantics changed.

Validation

• ./gradlew test
• ./gradlew check
• Added/updated tests for new behavior

Documentation

• Updated README (if needed)
• Added migration notes for breaking changes (if any)