chore: bump version to 3.5.0

[Diego Barajas (barajas-d)]

Summary

• Bump version from `3.4.0` to `3.5.0`
• Update `pom.xml`, `MercadoPagoConfig.java`, `README.md`, `CHANGELOG.md`

Changelog entry

Added

• Automatic Payments example: two-step recurring flow (#386)
• Order model: reuse `client/common` types in `OrderPayerRequest`, add `country` to `AddressRequest`, restore `currency` field position in `OrderCreateRequest` (#386)

Fixed

• Stored credential: rename `prevTransactionRef` to `previousTransactionReference` (#386)
• Deprecated `Matchers` replaced with `ArgumentMatchers` in unit tests (#386)

CI

• Standardize CI/CD workflows (#399)
• Add mock-based unit tests (#399)
• Skip lombok-maven-plugin delombok in CI to support Java 17 and 21 (#399)

Test plan

• CI passes
• Version string is correct in all files