release: update version 3.1.0 (#7900)

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[closes DevExpress/testcafe-private#229]

## Purpose
_Describe the problem you want to address or the feature you want to
implement._

## Approach
_Describe how your changes address the issue or implement the desired
functionality in as much detail as possible._

## References
_Provide a link to the existing issue(s), if any._

## Pre-Merge TODO
- [ ] Write tests for your proposed changes
- [ ] Make sure that existing tests do not fail

.publishrc

@@ -8,7 +8,7 @@
     "gitTag": true
   },
   "confirm": false,
-  "publishTag": "rc",
+  "publishTag": "latest",
   "prePublishScript": "gulp test-server",
   "postPublishScript": "gulp docker-publish"
 }

package.json

@@ -2,7 +2,7 @@
   "name": "testcafe",
   "description": "Automated browser testing for the modern web development stack.",
   "license": "MIT",
-  "version": "3.1.0-rc.3",
+  "version": "3.1.0",
   "author": {
     "name": "Developer Express Inc.",
     "url": "https://www.devexpress.com/"