pkg/labels/cidr: Optimize maskedIPToLabelString()
pkg/labels/cidr: Optimize maskedIPToLabelString()

Use strings.Builder instead of fmt.Sprintf() which is known to be not
performant.

This optimization was identified by inspecting a pprof (memory profile)
of a cluster with a wildcard L7 DNS rule policy (matchPattern: '*') and
a workload selected by the policy making a large amount of unique DNS
requests.

```
$ go test -v -run '^$' -bench 'Benchmark_maskedIPNetToLabelString' -benchtime 50000x -benchmem ./pkg/labels/cidr > old.txt
$ go test -v -run '^$' -bench 'Benchmark_maskedIPNetToLabelString' -benchtime 50000x -benchmem ./pkg/labels/cidr > new.txt
$ benchcmp old.txt new.txt
benchcmp is deprecated in favor of benchstat:
https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark                                old ns/op     new ns/op     delta
Benchmark_maskedIPNetToLabelString-8     8122          4450          -45.21%

benchmark                                old allocs     new allocs     delta
Benchmark_maskedIPNetToLabelString-8     39             32             -17.95%

benchmark                                old bytes     new bytes     delta
Benchmark_maskedIPNetToLabelString-8     488           368           -24.59%
```

Signed-off-by: Chris Tarazi <711c73f64afdce07b7e38039a96d2224209e9a6c@isovalent.com>

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