Update supported nvim versions

[hsanson]

Deprecate neovim 0.6 and support 0.11+

• Update default g:ale_use_neovim_diagnostics_api variable to has('nvim-0.7').
• Update ale-lint-file-linters documentation to mention 0.7+ instead of 0.6+
• Update Dockerfile to use 0.7.0 instead of 0.6.0
• update engine.vim check for has nvim-0.6.0
• update run-tests to use v0.7 and 0.10 instead of 0.6 and 0.9
• update ale-development.txt to mention 0.7.0 instead of 0.6.0
• update ale.vim to mention 0.7.0 instead of 0.6.0

[hsanson]

Unfortunately the docker images used to run tests is unable to build neovim-0.9 or newer making this change more challenging than expected: Vimjas/vim-testbed#91

We can consider increasing the minimum to 0.7 and leave tests up to 0.8 instead of latest 0.10.