[JENKINS-59291] upgrade to sse-gateway 1.21

[olamy]

Signed-off-by: olivier lamy [email protected]

Description

Upgrade sse-gateway 1.21 to fix issue for Windows/Edge
See JENKINS-59291.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given