package/mongodb: security bump to version 4.2.8

package/mongodb: security bump to version 4.2.8

Fix the following security issues:
- SERVER-45514 [FLE] Reject document validators with encryption-related
  keywords if the validationAction is “warn”
- SERVER-48039 Unrecognized option: net.ssl.clusterCertificateSelector
  in MongoDB v4.2
- SERVER-45803 mongodecrypt needs a ServiceContext
- SERVER-46834 Use monotonic time in UserCacheInvalidator
- SERVER-47113 LDAP connection pool acquisition state should own host
  list

https://docs.mongodb.com/manual/release-notes/4.2

Also:
- Update indentation in hash file (two spaces)
- Tweak version to be "compliant" with https://release-monitoring.org
- Use official tarball

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>