(MODULES-8830) Release mergeback 8.1.0

The Changelog links assumed the version was tagged as `v8.1.0` while it is `8.1.0`

Fixed Changelog links for 8.1.0

Reverting this change as the deepmerge function in stdlib does not work
the same as the mysql function. The mysql config is generated with
incorrect duplicate entries in which '-' and '_' are mixed up when
using the stdlib function.

pdksync - (MODULES-8444) - Raise lower Puppet bound

(MODULES-8886) Revert removal of deepmerge function

At this Time, this is not possible because a dupplicate resource error
occurs. With this change it is possible to generate a daily xtrabackup
and also a weekly mysqldump for example.

pdksync - (maint) Update pdk-template to f778803

Add a flag for activating or deactivating incremental backups. The
default is true so we don't change anything if you don't use the flag.
If the flag isi false we create a full backup every day instead of one
full backup and six incremental backups a week.

In addition, I outsourced the test of the mysql::backup::xtrabackup
class to test the additional parameters.

…tivable

Make incremental backups deactivable

Allow multiple backupmethods

(MODULES-9017) - update metadata to support current version of stdlib

Fix the contribution guide URL

MODULES-9160 - release 9.0.0