Update to standard github label: "enhancement" (#5997)

New github repos are populated with a defualt "enhancement" label.
We have used "Proposal" in our repo.
Aligning to the standard label  will help our unified SLA toolchain.

.github/ISSUE_TEMPLATE/community-module-support-request.md

@@ -2,7 +2,7 @@
 name: Community Module support request
 about: Suggest that Windows support be added to a community module
 title: Support react-native-foo
-labels: Extensions, Proposal
+labels: Extensions, enhancement
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/feature-request.md

@@ -2,7 +2,7 @@
 name: Feature request
 about: Suggest a new feature or idea
 title: Your feature request
-labels: Proposal
+labels: enhancement
 assignees: ''
 
 ---