replace strings.Split(N) for strings.Cut() or alternatives

Go 1.18 and up now provides a strings.Cut() which is better suited for
splitting key/value pairs (and similar constructs), and performs better:

```go
func BenchmarkSplit(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _ = strings.SplitN(s, "=", 2)[0]
                }
        }
}

func BenchmarkCut(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _, _, _ = strings.Cut(s, "=")
                }
        }
}
```

    BenchmarkSplit
    BenchmarkSplit-10            8244206               128.0 ns/op           128 B/op          4 allocs/op
    BenchmarkCut
    BenchmarkCut-10             54411998                21.80 ns/op            0 B/op          0 allocs/op

While looking at occurrences of `strings.Split()`, I also updated some for alternatives,
or added some constraints; for cases where an specific number of items is expected, I used `strings.SplitN()`
with a suitable limit. This prevents (theoretical) unlimited splits.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/container_log_test.go

@@ -141,10 +141,10 @@ func checkContainerLog(t *testing.T, log string, messages []string) {
 	lines := strings.Split(strings.TrimSpace(log), "\n")
 	require.Len(t, lines, len(messages), "log line number should match")
 	for i, line := range lines {
-		parts := strings.SplitN(line, " ", 2)
-		require.Len(t, parts, 2)
-		_, err := time.Parse(time.RFC3339Nano, parts[0])
+		ts, msg, ok := strings.Cut(line, " ")
+		require.True(t, ok)
+		_, err := time.Parse(time.RFC3339Nano, ts)
 		assert.NoError(t, err, "timestamp should be in RFC3339Nano format")
-		assert.Equal(t, messages[i], parts[1], "log content should match")
+		assert.Equal(t, messages[i], msg, "log content should match")
 	}
 }