Rename L2TP IPsec Settings dialog from Advanced to IPsec

Summary:
Calling the IPsec dialog Advanced is confusing, so rename it to IPsec.
Also in a subsequent patch there is also an Advanced group box in the IPsec dialog which would have added more confusion..

The following git file name renames occur in the vpn/l2tp folder and are in the patch file.
```
git mv l2tpadvancedwidget.cpp l2tpipsecwidget.cpp
git mv l2tpadvancedwidget.h l2tpipsecwidget.h
git mv l2tpadvanced.ui l2tpipsec.ui
```

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27763