BETA - Update to sentence case (#16590)

app/_locales/en/messages.json

@@ -438,7 +438,7 @@
     "message": "Beta"
   },
   "betaHeaderText": {
-    "message": "This is a BETA version. Please report bugs $1",
+    "message": "This is a beta version. Please report bugs $1",
     "description": "$1 represents the word 'here' in a hyperlink"
   },
   "betaMetamaskDescription": {

app/build-types/beta/manifest/chrome.json

@@ -1,4 +1,4 @@
 {
   "description": "THIS IS THE METAMASK EXTENSION BETA, INTENDED FOR BETA TESTING",
-  "name": "MetaMask BETA"
+  "name": "MetaMask Beta"
 }

ui/components/app/beta-header/__snapshots__/beta-header.test.js.snap

@@ -10,7 +10,7 @@ exports[`Beta Header should match snapshot 1`] = `
     >
       <span>
          
-        This is a BETA version. Please report bugs 
+        This is a beta version. Please report bugs 
         <a
           href="https://metamask.zendesk.com/hc/en-us"
           rel="noreferrer noopener"