[INSTALL]: cylc-flow@8.6.x to fix memory leak in 8.4.2

[climbfuji]

Package name

cylc-flow

Package version/tag

8.6.x

Build options

current

Installation timeframe

For spack-stack-2.2.0

Other information

There is a memory leak in cylc-flow@8.4.2 that was fixed in 8.6.0, see cylc/cylc-flow#7206 and https://github.com/cylc/cylc-flow/blob/master/CHANGES.md.

We should update cylc-flow to the latest patch release of 8.6.x in spack-stack-2.2.0

One additional compilication here is that cylc@8.6.0 and later require python@3.12. spack-stack uses python@3.11.11, but since cylc is in its own environment we may get by with just updating Python for the cylc-dev template.

Or we finally bite the bullet and update cylc-flow to 8.6.x, Python to 3.12.y, py-numpy to 2.whateverthelatest.