Rename `.sig` files to `.asc`: match upstream Go

[dagood]

Upstream Go provides .asc files, serving the same purpose as our .sig files:

https://golang.org/dl/go1.17.linux-amd64.tar.gz
https://golang.org/dl/go1.17.linux-amd64.tar.gz.asc

https://dotnetbuildoutput.blob.core.windows.net/golang/microsoft/main/20210818.1/go.20210818.1.linux-amd64.tar.gz.sig

They are both armored PGP signatures.

If I knew Go provided .asc files, I would have followed the naming, but they aren't advertised on the download page. I found out about them because the official golang Dockerfiles use the .asc files to verify the download.

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Changing to .asc would also let us remove some tweaks I made to the golang Dockerfiles that make them use .sig when downloading our builds.

[dagood]

Some recent thoughts on this from #2348 (comment), re: this being a breaking change at this point:

I think that publishing both .sig and .asc makes sense. We can then use .asc in our own infra to reduce patching. I don't know if it ever makes sense to stop publishing .sig, though.

.sig is actually a more intuitive extension to slap on our download URLs to get a signature (depending on exposure to other projects, etc.). It might actually be better to treat .asc as a "compatibility" signature, not even the main one.