Drop support for MongoDB v1

[thekid]

This pull request simplifies the code base by dropping backwards compatibility with MongoDB version 1.X

Changes

• Fold CollectionV2/CollectionV1 into TestingCollection
• Change from running findAndModify to using Collection::modify(), see Implement Collection modify() and remove() via findAndModify mongodb#45
• Remove support for old session layout with keys in root, see Refactor document substructure  #8
• Because this a BC break anyway, drop PHP < 7.4 at the same time, see Drop PHP 7 support xp-framework/rfc#343 (comment)

Effects

Before:

github.com/AlDanial/cloc v 1.82  T=0.03 s (105.4 files/s, 9975.4 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
PHP                              3             38            104            142
-------------------------------------------------------------------------------
SUM:                             3             38            104            142
-------------------------------------------------------------------------------

After:

github.com/AlDanial/cloc v 1.82  T=0.03 s (91.7 files/s, 7882.0 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
PHP                              3             34            103            121
-------------------------------------------------------------------------------
SUM:                             3             34            103            121
-------------------------------------------------------------------------------