docs: align README install versions with 5.3.1

[devondragon]

Bumps the README Maven/Gradle install snippets from 5.3.0 to 5.3.1 following the 5.3.1 release. The compatibility matrix already reads 5.3.x and the CHANGELOG 5.3.1 section is correct, so no other changes were needed.

Docs only, no runtime changes.

[claude]

Reviewed. This is a clean, minimal docs-only change:

• All three README references to 5.3.0 were updated to 5.3.1 consistently (Maven snippet, Gradle snippet under Spring Boot 4.x, and the Gradle snippet under the Spring Boot 4.0/4.1 setup steps).
• Verified no stray 5.3.0 references remain in README.md.
• No runtime/code changes, so no test coverage, security, or performance concerns apply here.

LGTM — no issues found.

[Copilot]

Pull request overview

Updates the README installation snippets to reflect the 5.3.1 release, keeping the documentation aligned with the current published framework version for Spring Boot 4.x consumers.

Changes:

• Updated the README Maven dependency snippet from 5.3.0 to 5.3.1.
• Updated the README Gradle dependency snippets from 5.3.0 to 5.3.1 (Installation + Quick Start).

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