Rename master to main

[chalin]

This is following most other CNCF repos. For details see:

• Renaming the default branch from master
• https://inclusivenaming.org
• Cloud Native Computing Foundation’s Inclusive Naming Initiative

We've done so recently for in-toto:

• Rename master to main in-toto/in-toto.io#42

@lukpueh - I'm glad to handle the rename here too if you give a thumbs up.

[lukpueh]

@lukpueh - I'm glad to handle the rename here too if you give a thumbs up.

Appreciated!

[chalin]

Done. Note to all: you'll need to update your local fork if you have one. For details:

[chalin]

Confirming that the builds work over main, see https://app.netlify.com/sites/theupdateframework/deploys/66d79a30a7edd4df6d70d7e4: