Update 6.9-to-7.0.md

fix gitlab.yml example diff command
Project labels tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add test for createing issue with labels

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Text changes to projects API documentation

Fixes some grammar / punctuation errors and inconsistencies.
Fixed typos in Development - Rake Tasks doc
Merge pull request #7411 from robbytaylor/patch-3

Fixed typos in Development - Rake Tasks doc
Merge pull request #7410 from robbytaylor/patch-1

Text changes to projects API documentation
Merge pull request #7201 from nacengineer/patch-2

Update 6.9-to-7.0.md
Replace invalid emojis
Mention the @all GFM reference
Merge pull request #7419 from Razer6/fix_emoji

[DOC] Replace invalid emojis
Merge pull request #7420 from Razer6/mention_all_markup

[DOC] Mention the @all GFM reference in documentation