Prepare v6.0.0 release (1st PS 0.15.0-compatible release)

[JordanMartinez]

Description of the change

Backlinking to purescript/purescript#4244. Prepares project for first release that is compatible with PureScript v0.15.0.

:ping_pong: @thomashoneyman

:robot: This is an automated pull request to prepare the next release of this library. PR was created via the Release.purs file. Some of the following steps are already done; others should be performed by a human once the pull request is merged:

• [x] Updated bower dependencies to 0.15.0-compatible versions
• [x] ci.yml: Node already set to 14.
• [x] ci.yml: Spago tests uncommented.
• [x] Updated changelog
• [ ] Publish a GitHub release.
• [ ] Upload the release to Pursuit with pulp publish.