Enable incremental webpack loading by default

Overview

With !52350 (merged) we introduced opt-in incremental webpack loading for development. It compiles much faster and uses less RAM. We should probably enable it by default in development mode with webpack-dev-server.

How to enable it currently

1. Make sure that gdk is up to date,
2. In your gdk.yml simply set:

   webpack:
     incremental: true

3. Run gdk reconfigure

How does it look

Implementation plan

• !68956 (merged) Add logic to IncrementalWebpackCompiler to only return entry points accessed within the 30 days
• gitlab-development-kit!2146 (merged) Change the default webpack.incremental value in the GDK to true.
• gitlab-development-kit!2146 (merged) Add webpack.incremental_ttl configuration to gdk.yml.
• Make sure the webpack-dev-server job in CI does not use the incremental compiler. Perhaps nothing needs to be done here, as the default enabled configuration will be done in the GDK. (See job https://gitlab.com/gitlab-org/gitlab/-/jobs/1533385290)
• Announce in #development, and cross-post to #frontend and #backend (https://gitlab.slack.com/archives/C02PF508L/p1630684426090700)
• Add to/announce in #eng-week-in-review