Replace the MD5 hashing of images with xxHash

Note that we only use this for change detection.

The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:

```
name          old time/op    new time/op    delta
HashImage-10    9.45µs ±21%   10.89µs ± 1%     ~     (p=0.343 n=4+4)

name          old alloc/op   new alloc/op   delta
HashImage-10      144B ± 0%        8B ± 0%  -94.44%  (p=0.029 n=4+4)

name          old allocs/op  new allocs/op  delta
HashImage-10      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)
```

tpl/hash/hash.go

@@ -16,10 +16,9 @@ package hash
 
 import (
 	"context"
-	"encoding/hex"
 	"hash/fnv"
 
-	"github.com/cespare/xxhash/v2"
+	"github.com/gohugoio/hugo/common/hashing"
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/tpl/internal"
 	"github.com/spf13/cast"
@@ -51,14 +50,7 @@ func (ns *Namespace) XxHash(v any) (string, error) {
 		return "", err
 	}
 
-	hasher := xxhash.New()
-
-	_, err = hasher.WriteString(conv)
-	if err != nil {
-		return "", err
-	}
-	hash := hasher.Sum(nil)
-	return hex.EncodeToString(hash), nil
+	return hashing.XxHashFromStringHexEncoded(conv), nil
 }
 
 const name = "hash"