tool: upgrade to go 1.20

[KevFan]

As a pre-req to #203, we should upgrade to Go 1.20

Things to consider on this upgrade:

• Workflows should be updated to this version
• Consider any functions that can be simplified from new functions natively provided by Go 1.20
• Any go.mod / go.sum changes

[KevFan]

@guicassolato do you have any thoughts on whether we want to use the experimental slices package (https://pkg.go.dev/golang.org/x/exp@v0.0.0-20230817173708-d852ddb80c63/slices) to remove / simplify https://github.com/Kuadrant/kuadrant-operator/blob/main/pkg/common/common.go#L92-L189

This slice package is provided as part of Go 1.21 standard library https://pkg.go.dev/slices@go1.21.0 but is not available in Go 1.20

[guicassolato]

@guicassolato do you have any thoughts on whether we want to use the experimental slices package

It's a shame it doesn't include a few generic functions we depend on (e.g. common.Map, common.Filter, common.Intersect, common.Intersection), but many others are there. So let's use it!