Upgrade gatsby to v4

[zasgar]

No description provided.

[NickLanam]

Gatsby v4 release notes.

The rewards for upgrading include:

• Parallel building of content. ~40% faster builds.
• Server-side rendering! Takes some effort to enable.
• Defer building infrequently-read pages until someone loads them (relies on SSR).
• A silly number of bugfixes and performance improvements.

Gatsby's developers also prepared an upgrade guide. The highlights:

• The final v3 release adds deprecation warnings to make it easier to transition in multiple steps.
• There does not seem to be automated tooling to handle most of the changes.
• Minimum node version updated (we already took care of this).
• Several plugins and configurations deprecated; numerous signature changes.

[ddelnano]

This would also address #264.