x/build: update OpenBSD builders to 7.6

[4a6f656c]

OpenBSD 7.6 has just been released (https://www.openbsd.org/76.html) - this means that the only two supported OpenBSD releases are now 7.5 and 7.6. The current openbsd-amd64 and openbsd-386 builders are running 7.2 which is unsupported. As such, these should be updated to run 7.6.

[gabyhelp]

Related Issues and Documentation

• x/build/env/openbsd-{386,amd64}: add an OpenBSD 7.0 (or newer) builder #48977 (closed)
• x/build/dashboard: add OpenBSD 7.3 and 7.4 builders (for 386, amd64) #63568
• x/build/dashboard: add OpenBSD 7.1 and 7.2 builders (for 386, amd64) #57496 (closed)
• builders: upgrade OpenBSD builders to 6.0 #16951 (closed)
• x/build: upgrade openbsd builders to 6.2 #22835 (closed)
• x/build/dashboard: remove OpenBSD 6.4 builders #49090 (closed)
• x/build: add OpenBSD 6.4 builder #26833 (closed)
• x/build/env/openbsd: add OpenBSD 6.8 builder #35712 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[4a6f656c]

Some useful details also in https://github.com/golang/go/issues/63568#issuecomment-1776088871

[gopherbot]

Change https://go.dev/cl/618895 mentions this issue: env/openbsd-amd64: update build image script to OpenBSD 7.6