Change default branch from `master` to `main`

[ronaldtse]

Description

It is now customary to use main instead of master as the default branch.

[ronaldtse]

@ribose-jeffreylau @ni4 @antonsviridenko any precautions we should take before the change?

[ni4]

All dependencies (like oss-fuzz, brewfile, whatever else?) which were sticked to default master naming could be affected. Probably we should re-check and build a list of things which should be updated.

[ronaldtse]

The downstream packaging repos need to be updated, but I believe they use the latest tag versions:

• https://github.com/rnpgp/homebrew-rnp
• https://github.com/rnpgp/chocolatey-rnp
• https://github.com/rnpgp/gentoo-rnp
• https://github.com/rnpgp/rnp-snap

[maxirmx]

• [x] https://github.com/rnpgp/rnp/oss-fuzz ----- uses default branch
• [ ] https://github.com/rnpgp/rnp/py-rnp ----- will be ok after https://github.com/rnpgp/py-rnp/pull/8
• [x] https://github.com/rnpgp/rnp/ruby-rnp ----- will be ok after https://github.com/rnpgp/ruby-rnp/pull/89
• [x] https://github.com/rnpgp/rnp/php-rnp ----- will be ok after https://github.com/rnpgp/php-rnp/pull/10
• [x] https://github.com/rnpgp/rnp/chocolatey-rnp ----- uses hardcoded 0.13.1 release tag, not branch name
• [x] https://github.com/rnpgp/rnp/gentoo-rnp ----- uses release tag, not branch name
• [x] https://github.com/rnpgp/rnp/rnp-objective-c ----- has 0.15.2 dylib checked in
• [x] https://github.com/rnpgp/rnp/homebrew-rnp ----- uses hardcoded 0.15.2 release tag, not branch name
• [x] https://github.com/rnpgp/rnp/snap-rnp ----- uses the last commited tag, not branch
• [x] https://github.com/rnpgp/rnp/rust-rnp ----- empty
• [x] https://github.com/rnpgp/rnp/go-rnp ----- empty

[ronaldtse]

The default branch is now changed to main.