Prepare for Apache Mynewt 1.15.0 release

Author: sjanc

RELEASE_NOTES.md

@@ -1,6 +1,6 @@
 # RELEASE NOTES
 
-15 December 2025 - Apache Newtmgr v1.14.0
+10 July 2026 - Apache Newtmgr v1.15.0
 
 For full release notes, please visit the
 [Apache Mynewt Wiki](https://cwiki.apache.org/confluence/display/MYNEWT/Release+Notes).

newtmgr/newtmgr.go

@@ -77,7 +77,7 @@ func main() {
 		ExeName:       "newtmgr",
 		ShortName:     "Newtmgr",
 		LongName:      "Apache Newtmgr",
-		VersionString: "1.15.0-dev",
+		VersionString: "1.15.0",
 		CfgFilename:   ".newtmgr.cp.json",
 	}