Revive the 1.5 maintenance baseline

[omandryk]

Summary

• replace retired Travis CI with a Java 8/11/17/21 GitHub Actions matrix
• update maintained dependencies, build plugins, project metadata, and contributor/security docs
• remove embedded WooCommerce credentials from the current integration-test source
• preserve the existing OAuth/signing and all other production behavior unchanged
• document the legacy HTTP OAuth limitation without claiming unsupported HTTPS authentication

Compatibility

This keeps the existing com.icoderman:wc-api-java coordinates, Java 8 bytecode target, public packages, and 1.x production code unchanged. The legacy Central deployment configuration is removed; a modern Central Portal release workflow will be added and validated separately before publishing 1.5.0.

Verification

mvn --batch-mode --no-transfer-progress clean verify -Dgpg.skip=true
Tests run: 7, Failures: 0, Errors: 0, Skipped: 7
BUILD SUCCESS

The seven skipped tests are the pre-existing live-store integration tests. Their embedded credentials were replaced with environment-variable lookups, but the tests remain explicitly disabled pending a disposable WooCommerce test environment.