Update supported browser versions

What does this MR do?

Make browserslist known as a frontend file

If we are adjusting browser versions, we want a full pipeline to see the effects (e.g. bundle size changes, etc.). Furthermore Danger should know about the browserslist file.

Ensure updating browsers forces babel to recompile

Given that we use @babel/preset-env transpiled files contain imports to core-js at the top of the file. babel-loader caches those transpiled files. If we bump the supported browser versions in .browserslistrc, we might need less imports of corejs modules. But as they are cached, we will retrieve those outdated files with the wrong imports causing unneccessary core-js modules to be imported.

Update supported browser versions

This increases supported browser versions to:

• Google Chrome/Chromium: at least major version 84 because older versions have an annoying bug with Array.reduce: https://bugs.chromium.org/p/chromium/issues/detail?id=1049982
• Microsoft Edge: at least majoy version 84 (dropping support for pre-Chromium Edge). (Same reasoning as with Chrome)
• Mozilla Firefox: at least major version 78, as this is the latest ESR and it is used quite a lot.
• Apple Safari: at least major version 13 (dropping support for Safari 12) because it is the second latest version of Safari

Changelog: changed

Update startup css

Due to our browser changes less vendor prefixes are needed, so we need to regenerate startup css files.

Update browser compatability data

It seems like the previous changes didn't yield the expecteded results and some polyfills were still included. By updating browser compatibility data we might be able to get newer information and hopefully less polyfills.

Increase assets cache key

Updating our supported browser versions is a major change to our assets. In order to ensure that there are no unintended side-effects, we update all caches.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team