Remove mention of NumFOCUS from the JCB charter

[jasongrout]

Also align some statements in the charter closer to how things have worked out in practice.

The JCB expects to continue iterating on the charter, but for now we have made smaller edits just focused on the sections mentioning NumFOCUS.

EC:

• [x] @fperez
• [x] @afshin
• [x] @jasongrout
• [x] @Ruv7
• [x] @Zsailer
• [x] @ellisonbg

JCB:

• [x] @jasongrout
• [x] @Ruv7
• [x] @marthacryan

[jasongrout]

CC @jupyter/executive-council for review

EC work item tracked at https://github.com/jupyter-governance/ec-team-compass/issues/62