chore: bump main lts to 7.0.0-SNAPSHOT (#2374)

Author: zhumin8

boms/cloud-lts-bom/pom.xml

@@ -7,7 +7,7 @@
 
   <groupId>com.google.cloud</groupId>
   <artifactId>gcp-lts-bom</artifactId>
-  <version>6.0.0-SNAPSHOT</version>
+  <version>7.0.0-SNAPSHOT</version>
   <packaging>pom</packaging>
 
   <name>Google Cloud Long Term Support BOM</name>