jenkins/plugins: make plugin versions compatible with Jenkins

The recent plugin version bumps breaks when building with the current
Jenkins version frozen in the cluster. Downgrade some of them to make
them compatible.

Fixes 436c1c8 ("jenkins/plugins: update plugins to latest versions").

Author: jlebon

jenkins/controller/plugins.txt

@@ -24,10 +24,10 @@
 basic-branch-build-strategies:81.v05e333931c7d
 generic-webhook-trigger:1.88.0
 github-oauth:597.ve0c3480fcb_d0
-kubernetes-credentials-provider:1.258.v95949f923a_a_e
+kubernetes-credentials-provider:1.225.v14f9e6b_28f53
 pipeline-github:2.8-155.8eab375ac9f8
 slack:684.v833089650554
-timestamper:1.26
+timestamper:1.25
 splunk-devops-extend:1.10.1
 splunk-devops:1.10.1
 antisamy-markup-formatter:162.v0e6ec0fcfcf6