Add People API upgrade instruction to upgrade guide (#8441)

I'm updating the docs as we now require the People API to be available
to use messaging sync. This has been reported by a user self-hosting the
app on discord.

packages/twenty-website/src/content/developers/self-hosting/upgrade-guide.mdx

@@ -127,5 +127,10 @@ We have also simplifed the way we handle the JWT tokens.
 - Added: `APP_SECRET`
 
 Update your `.env` file to use the new `APP_SECRET` variable instead of the individual tokens secrets (you can use the same secret as before or generate a new random string)
+*Note*: Keep the `ACCESS_TOKEN_SECRET` variable to keep the backward compatibility with the previous API tokens.
+
+### Connected Account
+
+If you are using connected account to synchronize your Google emails and calendars, you will need to activate the [People API](https://developers.google.com/people) on your Google Admin console.
 
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