Bump xstream version from 1.4.20 to 1.4.21 for CVE-2024-47072.

[CraigAtInstrumental]

Description

Bump xstream version from 1.4.20 to 1.4.21 for CVE-2024-47072.

Motivation and Context

Fixes CVE-2024-47072. Addresses link

How Has This Been Tested?

Ran 'gradlew check' and 'gradlew test'.

Screenshots (if appropriate):

Types of changes

Security vulnerability fix

Checklist:

• [X] My code follows the code style of this project.
• [X] I have updated the documentation accordingly.