Set releaseCandidate to actual null value, not 'null' string (#54929)

Author: Zenulous

src/versions/lib/enterprise-server-releases.js

@@ -15,7 +15,7 @@ export const nextNext = '3.18'
 export const supported = ['3.16', '3.15', '3.14', '3.13', '3.12']
 
 // Edit this to `null` when it's no longer the release candidate
-export const releaseCandidate = 'null'
+export const releaseCandidate = null
 
 // Ensure that:
 // "next" is ahead of "latest" by one minor or major release.