upgrade go to v1.21, enable image data piping

[kheina]

• upgrades go to v1.21 and eliminates the use of imax in favor of go's built-in max function
• imports gif and webp libraries to add support for those file types
• reworks cmd/thumbhash/main.go slightly to add support for piping image data directly to thumbhash encode-image for better interoperability

[galdor]

The first patch mixes the Go version update with other unrelated changes. if you send me a patch with just the bump to 1.21 (and you could even go to 1.22) and the use of the builtins min and max, I'll apply it.

For the second patch, it seems like a good idea but a quick read shows a couple minor issues (e.g. if the argument is now optional, why would you make it mandatory in readFilePath?). I'll see what I can do when I get the time.