[RFC] Move forward with PHP 5.3 and 7.x / 8.x

[serbanghita]

The goal of the project is still "detect the mobile environment and separate tablets from phones".

Here are the changes that I'm about to perform in order to move the project further.

Steps

1. devel branch is being deprecated and will move to branch 5.x. Content from 5.x will be mirrored to devel until we close all PRs and issues. From 5.x we will release tags 2.x.x for LTS of PHP 5.3
2. create branch 7.x for supporting PHP7. Releases will have tags 3.x.x.
3. create branch 8.x for supporting PHP8. Releases will have tags 4.x.x.

7.x and 8x changes

Mobile Detect for 7.x and 8.x will have some breaking changes:

• [x] remove mobileGrade() method. This is obsolete and serves no real purpose. Add a @deprecated warning to 5.x branch.
• [x] DETECTION_TYPE_MOBILE and methods setDetectionType, getMobileDetectionRulesExtended will be removed.
• [x] big arrays like SamsungTablet will be broken down in multiple arrays. TODO: evaluate memory and performance.

[serbanghita]

Created new branches:

• 5.x https://github.com/serbanghita/Mobile-Detect/tree/5.x (default) - for now.
• 7.4 https://github.com/serbanghita/Mobile-Detect/tree/7.4

When 7.4 will be stable, it will become the default branch.

[serbanghita]

Will open a separate issue for 7.4 official release.