How to enforce TLS v1.2 in Java program talking to Splunk server via log4j2 ?

[maciej_kw]

I am trying to get our Java client application talking to a Splunk server provided by one of our vendors.
The communication is supposed to be simple. Our client app will send just plain text messages once a while.
I am using log4j2 as a platform for communication with the Splunk server.
Below are Splunk related appender and logger defined in the log4j2.xml file.
Vendor claims they do not receive messages from us.
However they see some traffic originating from our server where the application runs.
They suggest to enforce TLS v1.2 protocol on communication with their Splunk server.
How can we do that?
log4j2 configuration:

<Appenders>
    <Http name="httpSplunk"
                url="...some.server.com:8088/services/collector/event"
                token="some-token"
                host="$${env:COMPUTERNAME}"
                index=""
                source=""
                sourcetype=""
                middleware=""
                batch_size_bytes="10240"
                batch_size_count="10"
                batch_interval="1000"
                disableCertificateValidation="true">
        <PatternLayout pattern="%m"/>
    </Http>
</Appenders>
<Loggers>
    <Root level="warn">
        <AppenderRef ref="httpSplunk"/>
    </Root>
    <Logger name="splunk.Ellenby" level="info">
        <AppenderRef ref="httpSplunk"/>
    </Logger>
</Loggers>

Thank you

Best Regards,
Maciej Kwiecinski