chore: bump plugin version to 3.15.13

[yxxhero]

Description

Bumps the plugin version to 3.15.13 in plugin.yaml.

Changes since v3.15.12

• feat: make --three-way-merge work without patch permissions (feat: make --three-way-merge work without patch permissions #1063)
• Bump Helm support matrix versions to v3.22.0 and v4.3.0 (Bump Helm support matrix versions to v3.22.0 and v4.3.0 #1069)
• fix: omit null/empty labels and annotations keys from diffs (fix: omit null/empty labels and annotations keys from diffs (#1064) #1068)
• fix: strip fluxcd managed labels (fix: strip fluxcd managed labels #1065)
• build(deps): bump helm/kind-action from 1.14.0 to 1.15.0 (build(deps): bump helm/kind-action from 1.14.0 to 1.15.0 #1066)
• build(deps): bump k8s.io/cli-runtime from 0.36.4 to 0.37.0 (build(deps): bump k8s.io/cli-runtime from 0.36.4 to 0.37.0 #1061)
• build(deps): bump k8s.io/apiextensions-apiserver from 0.36.4 to 0.37.0 (build(deps): bump k8s.io/apiextensions-apiserver from 0.36.4 to 0.37.0 #1058)
• build(deps): bump github.com/stretchr/testify from 1.11.1 to 1.12.1 (build(deps): bump github.com/stretchr/testify from 1.11.1 to 1.12.1 #1062)