Checked new version of patch attached here https://bugs.launchpad.net/swift/+bug/1998625/comments/23, looks ok.

Before patch:
```
vagrant@vagrant:~/swift$ swift-init restart proxy
Signal proxy-server  pid: 223087  signal: Signals.SIGTERM
Signal proxy-server  pid: 223088  signal: Signals.SIGTERM
proxy-server (223088) appears to have stopped
proxy-server (223087) appears to have stopped
WARNING: Unable to modify max process limit.  Running as non-root?
Starting proxy-server...(/etc/swift/proxy-server/proxy-noauth.conf.d)
Starting proxy-server...(/etc/swift/proxy-server/proxy-server.conf.d)

vagrant@vagrant:~/swift$ curl 'http://saio3:8080/my-bucket?acl=' -X PUT -H "Host: saio3:8080" -H "Content-Type: application/x-www-form-urlencoded; charset=utf-8" -H "X-Amz-Content-Sha256: 4fd507f2889d8d42ee3e03fb20860748c0e5921dc345676d85fae0cc5891dfdf" -H "X-Amz-Date: 20230113T123756Z" -H "Authorization: AWS4-HMAC-SHA256 Credential=test:tester/20230113/us-east-1/s3/aws4_request, SignedHeaders=content-length;content-type;host;x-amz-content-sha256;x-amz-date, Signature=054202b4db5e2d6e047f7abc6964555bb36538a265399bfdcff13bbe32c1e95b" --data-binary '<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/hostname"> ]>
<AccessControlPolicy xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
<Owner>
    <DisplayName>test:tester</DisplayName>
    <ID>test:tester</ID>
</Owner>
<AccessControlList>
    <Grant>
        <Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
            <DisplayName>foo &xxe;</DisplayName>
            <ID>foo &xxe;</ID>
        </Grantee>
        <Permission>WRITE</Permission>
    </Grant>
</AccessControlList>
</AccessControlPolicy>'

vagrant@vagrant:~/swift$ aws s3api get-bucket-acl --bucket my-bucket
{
    "Owner": {
        "DisplayName": "test:tester",
        "ID": "test:tester"
    },
    "Grants": [
        {
            "Grantee": {
                "DisplayName": "foo vagrant\n",
                "ID": "foo vagrant\n",
                "Type": "CanonicalUser"
            },
            "Permission": "WRITE"
        }
    ]
}
```

After patch:
```
vagrant@vagrant:~/swift$ swift-init restart proxy
Signal proxy-server  pid: 223110  signal: Signals.SIGTERM
Signal proxy-server  pid: 223111  signal: Signals.SIGTERM
proxy-server (223110) appears to have stopped
proxy-server (223111) appears to have stopped
WARNING: Unable to modify max process limit.  Running as non-root?
Starting proxy-server...(/etc/swift/proxy-server/proxy-noauth.conf.d)
Starting proxy-server...(/etc/swift/proxy-server/proxy-server.conf.d)

vagrant@vagrant:~/swift$ curl 'http://saio3:8080/my-bucket?acl=' -X PUT -H "Host: saio3:8080" -H "Content-Type: application/x-www-form-urlencoded; charset=utf-8" -H "X-Amz-Content-Sha256: 4fd507f2889d8d42ee3e03fb20860748c0e5921dc345676d85fae0cc5891dfdf" -H "X-Amz-Date: 20230113T123756Z" -H "Authorization: AWS4-HMAC-SHA256 Credential=test:tester/20230113/us-east-1/s3/aws4_request, SignedHeaders=content-length;content-type;host;x-amz-content-sha256;x-amz-date, Signature=054202b4db5e2d6e047f7abc6964555bb36538a265399bfdcff13bbe32c1e95b" --data-binary '<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/hostname"> ]>
<AccessControlPolicy xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
<Owner>
    <DisplayName>test:tester</DisplayName>
    <ID>test:tester</ID>
</Owner>
<AccessControlList>
    <Grant>
        <Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
            <DisplayName>foo &xxe;</DisplayName>
            <ID>foo &xxe;</ID>
        </Grantee>
        <Permission>WRITE</Permission>
    </Grant>
</AccessControlList>
</AccessControlPolicy>'

vagrant@vagrant:~/swift$ aws s3api get-bucket-acl --bucket my-bucket
{
    "Owner": {
        "DisplayName": "test:tester",
        "ID": "test:tester"
    },
    "Grants": [
        {
            "Grantee": {
                "DisplayName": "foo ",
                "ID": "foo ",
                "Type": "CanonicalUser"
            },
            "Permission": "WRITE"
        }
    ]
}
```

Checked the unit tests fail if fix is reverted.

Unable to confirm the functional test due to something awry with boto on my vsaio (not specific to the new functional test).