Homebrew's default mbedtls is now 4.x, so switch the macOS build and CI job
to it (drop the mbedtls@3 pin). That leaves 3.x (Ubuntu 24.04 apt ships 2.28,
macOS now 4.x) uncovered, so add an ubuntu-26.04 job whose apt provides Mbed
TLS 3.6. Net coverage: 2.28 (ubuntu-latest), 3.6 (ubuntu-26.04), 4.2 (macOS).
ubuntu-26.04 is a public-preview runner image; fold it into the main ubuntu
matrix once ubuntu-latest moves to 26.04.