Fix deprecation: render :text is deprecated because it does not actually render a text/plain response

Enables frozen string for the following:

* app/controllers/dashboard/**/*.rb
* app/controllers/explore/**/*.rb
* app/controllers/google_api/**/*.rb
* app/controllers/groups/**/*.rb
* app/controllers/import/**/*.rb
* app/controllers/instance_statistics/**/*.rb
* app/controllers/ldap/**/*.rb
* app/controllers/oauth/**/*.rb
* app/controllers/profiles/**/*.rb

Partially addresses #47424.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Add a rubocop rule to check if a method 'redirect_to' is used without explicitly set 'status' in 'destroy' actions of controllers

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Report: https://github.com/gitlabhq/gitlabhq/issues/10138



Signed-off-by: Rémy Coutable <remy@rymai.me>

`render nothing: true` has been deprecated.
For more information see [pr](https://github.com/rails/rails/pull/20336)

Closes #13860

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

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

Currently this method responds with `text/html`. It is kind of unusable
if you open it in a browser. The browser thinks it is HTML and renders
it as HTML, meaning new lines are dropped. So it's very hard to
distinguish where the key starts and where it ends.

This commit changes the content type header to `text/plain`.

before:
GET /user.keys

ssh-rsa ...\nssh-rsa ...\nssh-rsa ...

after:
GET /user.keys

ssh-rsa ...
ssh-rsa ...
sha-rsa ...

getting user keys publically through http without any authentication, the github way. E.g: http://github.com/devaroop.keys

changelog updated to include ssh key retrieval feature update

getting user keys publically through http without any authentication, the github way. E.g: http://github.com/devaroop.keys

Didn't bother with files in db/, config/, or features/