Update link for Scala Code of Conduct

[SgtSwagrid]

The previous link was broken, directing people to https://github.com/scala-steward-org/scala-steward/blob/master/CODE_OF_CONDUCT.md while the master branch seems to no longer exist.

[rtyley]

the master branch seems to no longer exist.

Yeah, the master branch has been renamed to main, which redirects on GitHub, so

https://github.com/scala-steward-org/scala-steward/blob/master/CODE_OF_CONDUCT.md

will link to this file, which does exist:

https://github.com/scala-steward-org/scala-steward/blob/main/CODE_OF_CONDUCT.md

The redirection for master to main works on GitHub, but possibly not in other environments - for those environments, it probably makes sense to update the two links that use scala-steward/blob/master to scala-steward/blob/main - eg:

scala-steward/README.md

Line 254 in e6ce04e

[CoC]: https://github.com/scala-steward-org/scala-steward/blob/master/CODE_OF_CONDUCT.md

[SgtSwagrid]

I reverted the link to again use the existing CoC definition, and instead updated the definition itself.

Some notes:

• The link claims to go to the "Scala Code of Conduct", but it actually goes to the scala-steward code of conduct. This in turn has a link to the actual Scala Code of Conduct, so it's probably fine, but is technically inaccurate.

• At least for me, the previous link didn't work when clicked from the main page on GitHub (i.e. https://github.com/scala-steward-org/scala-steward), but does work when clicked from this PR, which is peculiar if nothing else.

[mzuehlke]

Please update his branch from master to main, too