Raise the baseline to Java 17

[jponge]

Let's consider this.

Flow has obviously not changed, but there might be a few places in the Mutiny implementation where language and standard library upgrades might be helpful.

/cc @heubeck @cescoffier

[jponge]

Target is before October 2024 (OpenJDK 11 EOL)

[heubeck]

Would support this.