doc,n-api: fix introduced_in metadata

Node.js v7.10.0 did not contain N-API. Update the `introduced_in`
metadata to prevent a broken 7.x "View another version" link in the
N-API docs.

PR-URL: #27745
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/n-api.md

@@ -1,6 +1,6 @@
 # N-API
 
-<!--introduced_in=v7.10.0-->
+<!--introduced_in=v8.0.0-->
 <!-- type=misc -->
 
 > Stability: 2 - Stable