Fix incorrect occurrences of dns where it should be dsn

[mollux]

Q	A
Bug fix? (use the a.b branch)	[x]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

in code there are multiple occurences of dns, where it should be dsn.
This PR fixes this

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. ensure assets are regenerated, as there are changes to the source js files
3. edit DSN settings for mailer and messenger via the UI, and verify all logic is still working

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (efb9ac1) 58.60% compared to head (366ab0c) 58.60%.

Additional details and impacted files

@@            Coverage Diff            @@
##                5.0   #13259   +/-   ##
=========================================
  Coverage     58.60%   58.60%           
  Complexity    32985    32985           
=========================================
  Files          2183     2183           
  Lines         98756    98756           
=========================================
  Hits          57877    57877           
  Misses        40879    40879           

Files	Coverage Δ
app/bundles/ConfigBundle/Form/Type/DsnType.php	92.30% <100.00%> (ø)
...p/bundles/MessengerBundle/Form/Type/ConfigType.php	100.00% <100.00%> (ø)
...dles/MessengerBundle/Controller/AjaxController.php	0.00% <0.00%> (ø)
...les/MessengerBundle/MessageHandler/TestHandler.php	26.66% <0.00%> (ø)

[mabumusa1]

Changes are good

[kuzmany]

Nice catch 👍