remove extra upgrade details, add missing config

Update 7.5 update guide:

* Remove unnecessary upgrade details (they were completed in 7.4 upgrade and only needed one time)
* Add missing Nginx config details
add details on backing up your SSH host keys

Users need to backup SSH host keys if they want to do a complete restore
to the same domain and not have users get the `WARNING: REMOTE HOST
IDENTIFICATION HAS CHANGED!` message.
Fix typo in the README.md for docker

The container name has been previously renamed to "gitlab_app".
Use shell invocation according to the shell commands guidelines.
Use system where only return result is needed.
Don't forget to splat.
Merge branch 'fix_shell_commands_in_shell_install' into 'master'

Fix shell commands in shell install

Fixes #1610

See merge request !1320
Add strict validation to snippet file names

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Sanitize snippet file name in raw headers

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Fix snippet factory

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'sanitize-snippet-file-name' into 'master'

Validate and sanitize snippet file name

Fixes #1816

See merge request !1322
Update CHANGELOG with snippet validation change

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge pull request #8359 from bbodenmiller/patch-1

add details on backing up your SSH host keys
Merge pull request #8429 from zertrin/fix-typo-in-docker-readme-1

Fix typo in the README.md for docker
Add missing webhook doc for tag event
Merge pull request #8374 from bbodenmiller/7-5-update-fix

remove extra upgrade details, add missing config
Merge pull request #8410 from tricknotes/add-webhook-doc-for-tag-event

Add missing webhook doc for tag event