docs: switch to crossplane-system namespace everywhere for consistency

[phisco]

Description of your changes

Switching all occurrences of upbound-system namespace back to crossplane-namespace.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

[bobh66]

crossplane-system is not created anymore,

Since when? Community crossplane deploys into crossplane-system, and that should remain the default for all non-Upbound providers.

upbound-system is only used for downstream uxp and should not be referenced in anything upstream of uxp

[turkenh]

Since when? Community crossplane deploys into crossplane-system, and that should remain the default for all non-Upbound providers.

upbound-system is only used for downstream uxp and should not be referenced in anything upstream of uxp

Agreed.

This was changed for local development recently for integrating uptest for e2e testing: #118

But, I believe we should rather configure uptest to use crossplane-system instead.

[phisco]

good point @bobh66, totally agree, my main concern was about consistency and the procedure in the README properly working. I switched everything to crossplane-system and tomorrow we'll fix the build sub module to properly handle CROSSPLANE_NAMESPACE 👍

[bobh66]

Thanks @phisco !

[turkenh]

Thanks @phisco 🙌

[phisco]

Just noticed the cleanup make local.down in the README doesn't work for me.
I'm not sure which would be the correct target to use for that though 🤔
CC @turkenh

[turkenh]

@phisco I assume it should be updated as make controlplane.down.

[phisco]

pushed the change, it works fine now, totally missed that target, thanks @turkenh!