docs: prepare changelog for v1.21.0 release

[czeslavo]

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[czeslavo]

https://github.com/Kong/deck/actions/runs/5120725557/jobs/9207582099?pr=924 This failure seems to be a race condition as tests are using the same Konnect runtime group.

[GGabriele]

@czeslavo Despite the changelog was bumped to 1.20.0, the actual release was never done. Would you mind fixing the changelog updating the 1.20.0 entry with the following:

• fix: return proper error when HTTP calls fail on validate #869
• fix: replace old docs link in 'convert' and fix docstring #905
• chore: bump Go to 1.20 #898

After that, we can merge and do a proper 1.20.0 this time.

[czeslavo]

@czeslavo Despite the changelog was bumped to 1.20.0, the actual release was never done. Would you mind fixing the changelog updating the 1.20.0 entry with the following:

• fix: return proper error when HTTP calls fail on validate #869
• fix: replace old docs link in 'convert' and fix docstring #905
• chore: bump Go to 1.20 #898

After that, we can merge and do a proper 1.20.0 this time.

Hmm, I'm not sure if we should do v1.20.0 release as the v1.20.0 tag already exists. I think it was just the release job that failed when the tag was pushed: https://github.com/Kong/deck/actions/runs/4791803501

[czeslavo]

I'm not sure what is the root cause of the release job failure for v1.20.0, but maybe we could try to rerun it? @GGabriele