[DOCS] Clarify cluster health status during rolling upgrade (#40757)

Remove mention of the `yellow` and `red` starting
health status from the rolling upgrade docs.

Instead, we should emphasize that users wait 
for the node to recover with a health status of
`green` rather than the starting status.

Co-authored-by: James Rodewig <[email protected]>

Author: GlenRSmith

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -142,8 +142,8 @@ You can check progress by submitting a <<cat-health,`_cat/health`>> request:
 GET _cat/health?v
 --------------------------------------------------
 
-Wait for the `status` column to switch from `yellow` to `green`. Once the
-node is `green`, all primary and replica shards have been allocated.
+Wait for the `status` column to switch to `green`. Once the node is `green`, all
+primary and replica shards have been allocated.
 
 [IMPORTANT]
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