CPBR-3030: Upgrade Python to 3.14 for 8.1.2-cp1

[Nitin Singh (nitsingh-ui)]

Summary

• Port Python 3.14 from-source build from PR Merge Conflict Resolution (from 7.9.x to 8.0.x) #1321 (8.0.x) to 8.1.2-cp1
• Fixes cp-base-new Docker build failure caused by python3-3.9.25-3.el9_7.1 RPM being removed from UBI9 repos
• Exact same changes as PR Merge Conflict Resolution (from 7.9.x to 8.0.x) #1321, adapted for this branch

Changes (10 files, matching PR #1321)

• base/Dockerfile.ubi9 — Replace yum-installed python3 with Python 3.14 compiled from source
• base/pom.xml — PYTHON39_VERSION → PYTHON314_VERSION build args
• pom.xml — Remove stale python3/python39 version pins, add python.python314.version=3.14.3
• .semaphore/cp_dockerfile_build.yml — sem-version python 3.9 → 3.14
• .semaphore/semaphore.yml — sem-version python 3.9 → 3.14
• base/requirements.txt — confluent-docker-utils v0.0.162 → v0.0.169
• base-java/requirements.txt — confluent-docker-utils v0.0.162 → v0.0.169
• base/tox.ini — pytest/sphinx version bumps for Python 3.14 compatibility
• base-java/tox.ini — pytest/sphinx version bumps for Python 3.14 compatibility
• service.yml — lang_version 3.9 → 3.14

Context

• The 8.1.2-cp1 branch was cut from 8.1.x before the Python 3.14 migration landed
• Red Hat removed python3-3.9.25-3.el9_7.1 from UBI9 repos, breaking the cp-base-new build
• PR Fix cp-base-new build: add explicit gzip dependency in ubi9 #1493 (gzip dependency fix) may be needed as a followup if this build fails

Test plan

• CI build passes on this branch
• cp-base-new Docker image builds successfully on both AMD and ARM