Brewfile: use openssl@3

@3.0 is old, @3 installs something recent (or
even that is installed already).

Brewfile

@@ -2,7 +2,7 @@ brew 'pkgconf'
 brew 'zstd'
 brew 'lz4'
 brew 'xxhash'
-brew 'openssl@3.0'
+brew 'openssl@3'
 
 # osxfuse (aka macFUSE) is only required for "borg mount",
 # but won't work on GitHub Actions' workers.