Merge branch 'update-supported-web-browsers' into 'master'

Update "Supported web browsers" text

## What does this MR do?

Updates the `Supported web browsers` text in the installation docs.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

Slack discussion

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7341

doc/install/requirements.md

@@ -143,9 +143,6 @@ On a very active server (10,000 active users) the Sidekiq process can use 1GB+ o
 
 ## Supported web browsers
 
-- Chrome (Latest stable version)
-- Firefox (Latest released version and [latest ESR version](https://www.mozilla.org/en-US/firefox/organizations/))
-- Safari 7+ (known problem: required fields in html5 do not work)
-- Opera (Latest released version)
-- Internet Explorer (IE) 11+ but please make sure that you have the `Compatibility View` mode disabled.
-- Edge (Latest stable version)
+We support the current and the previous major release of Firefox, Safari and Microsoft browsers (Microsoft Edge and Internet Explorer 11).
+
+Each time a new browser version is released, we begin supporting that version and stop supporting the third most recent version.