Fix: [GSTG] Decrease Thanos memcache max item size to 512KiB
After deploying !164 (merged) memcached containers failed to start with the error:
-I (item_size_max: 536576) must be evenly divisible by slab_chunk_max (bytes: 524288)
.
This MR updates all occurrences of 524K to 512K.