Upgrade sliding-sync (v0.99.14 -> v0.99.15)

roles/custom/matrix-sliding-sync/defaults/main.yml

@@ -6,7 +6,7 @@
 matrix_sliding_sync_enabled: true
 
 # renovate: datasource=docker depName=ghcr.io/matrix-org/sliding-sync
-matrix_sliding_sync_version: v0.99.14
+matrix_sliding_sync_version: v0.99.15
 
 matrix_sliding_sync_scheme: https