fix: Mark metric envelope items as deprecated (#12701)

Mark both statsd and metric meta envelope items as deprecated.

Author: philipphofmann

develop-docs/sdk/data-model/envelope-items.mdx

@@ -146,7 +146,7 @@ details.
 
 *None*
 
-### Statsd
+### Statsd (deprecated)
 
 Item type `"statsd"` contains metrics emissions in a superset of the statsd format.
 
@@ -162,7 +162,7 @@ details.
 
 *None*
 
-### Metric Meta
+### Metric Meta (deprecated)
 
 Item type `"metric_meta"` contains per-metric meta data which is persisted alongside
 metrics in the system.