Modernize to Java 17 and Spring Boot 3.3

[devin-ai-integration]

Summary

Upgrades the legacy Java 8 / Spring Boot 2.0.2 (2018) app to Java 17 / Spring Boot 3.3.13, verified end-to-end: mvn clean package succeeds on JDK 17 and the app boots with all /topic CRUD endpoints working against the in-memory H2 store.

Key changes beyond version bumps:

• pom.xml: packaging pom → jar (the old packaging skipped compilation entirely, so mvn package never produced a runnable jar); dropped spring-boot-properties-migrator.
• build.gradle: migrated from buildscript + compile/testCompile to the plugins DSL with implementation/testImplementation and a Java 17 toolchain.
• Application: replaced removed-in-Spring-6 jdbcTemplate.query(sql, Object[], RowMapper) with the varargs form; the startup quote fetch now targets the live quoters endpoint and degrades gracefully (log.warn) instead of crashing startup when unreachable (the old gturnquist-quoters.cfapps.io host is defunct).
• Stopped tracking target/ build output and fixed .gitignore.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/58e031ae89be4be3b6a19a88ff577c5a
Requested by: @joebensoncog97

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Devin errored when opening this Pull Request as joebensoncog97.
As a fallback, Devin opened this PR as itself.

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