Plans Grid: enable accordion view as non-experimental

[razvanpapadopol]

*Editing Toolkit release notes: N/A (package API update)

Changes proposed in this Pull Request

• Rename isExperimental prop to isAccordion for PlansGrid.
• update usages in Step by Step launch and New Onboarding.
• cleanup gutenboarding/feature-picker flag from the New Onboarding.

Testing instructions - nothing should change

• Go to /new and reach plans step or open Plans modal
• Accordion view should be displayed, as in production

[matticbot]

Test live: https://calypso.live/?branch=update/plans-grid-accordion-non-experimental

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D52859-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~47 bytes removed 📉 [gzipped])

name                 parsed_size           gzip_size
entry-gutenboarding       -286 B  (-0.0%)      -47 B  (-0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[tjcafferkey]

These changes LGTM and work as expected.

As a side note, I think we should try and track each PR with an Issue for more context and visibility.