[JENKINS-59291] upgrade to sse-gateway 1.21 (#2053)
[JENKINS-59291] upgrade to sse-gateway 1.21 (#2053)

Signed-off-by: olivier lamy <d3548cf353cfa8a03c6f2a815e39e2c0a78449a5@apache.org>
jenkinsci/blueocean-plugin,jenkinsci/blueocean-plugin,jenkinsci/blueocean-plugin,jenkinsci/blueocean-plugin,jenkinsci/blueocean-plugin