Reduce Java Heap for Mill Client

[gamlerhart]

The Mill Client only forwards requests and therefore does not need much memory.
However, Java doesn't know that an allocates a large heap based on the machines memory size.

Therefore, give a way lower heap limit,
creating a leaner client memory wise.

Example on my machine:

• Before: A Mill client reserved ~500MByte
• After: A Mill client reserves the specified 24Mbyte

This is mostly relevant for a long running mill client (examples --watch clients, -i --no-build-lock and --bsp`).

[gamlerhart]

facepalm. Its also added to the test-runner (mill.testrunner.entrypoint.TestRunnerMain), creating out of memory exceptions. Will take a look later.