Support Go 1.25

[jorendorff]

I had trouble running script/cibuild in part because script/ensure-go-installed contains a regexp that must match the user's Go version, and the regexp does not match go1.25.6 (the version I have). That could be fixed by adding a 5 to the regexp:

SUPPORTED_GO_VERSIONS='go1.2[012345]'

Separately, the script's behavior when the regexp doesn't match isn't super helpful. It tries to download and install a local copy of a version of Go, but on Mac this doesn't work. I'm not sure what's going wrong. The content of .vendor/golocal after the script runs is like this:

~/src/gh-ost/.vendor/golocal (cibuild) $ ls -lR
total 148264
-rw-r--r--  1 jorendorff  staff      1813 Feb 10 10:00 Distribution
-rw-r--r--  1 jorendorff  staff  75906569 Feb 10 10:00 go1.23.0.darwin-amd64.pkg
drwx------  6 jorendorff  staff       192 Dec 31  1969 org.golang.go.pkg
drwx------  3 jorendorff  staff        96 Dec 31  1969 Resources

./org.golang.go.pkg:
total 154328
-rw-r--r--  1 jorendorff  staff   3936029 Aug  7  2024 Bom
-rw-r--r--  1 jorendorff  staff       569 Feb 10 10:00 PackageInfo
-rw-r--r--  1 jorendorff  staff  75071153 Aug  7  2024 Payload
-rw-r--r--  1 jorendorff  staff       299 Aug  7  2024 Scripts

./Resources:
total 24
-rw-r--r--  1 jorendorff  staff  9552 Feb 10 10:00 background.png