Pin os-capacity to v0.5 release

[JohnGarbutt]

Version v0.3 was broken an automatically pulled
on a fresh controller. Lets start pinning this version.

Note v0.5 fixes 842 CVEs vs the previous version:
https://github.com/stackhpc/os-capacity/security/code-scanning

[Alex-Welsh]

Since it's arguably a bugfix, could we aim this at stackhpc/2023.1 first? We'll then merge it up from there.

I'd also prefer it if we could slurp this out to a variable and set it somewhere like stackhpc-monitoring.yml so it's a bit easier to find

Both of those suggestions are easy to do retroactively so if this is a more urgent fix I'm happy to merge now and patch later

[Alex-Welsh]

Looks good. For future reference, I would normally say we need to have release notes when we change pins but in this case, we're pinning it to the release it's already on so nothing functionally changes.