rpk: consolidate plugin version validation on pkg

Define the semver pattern once in pkg/redpanda:
VersionFromString parses versions from command
output, and a new ValidVersion strictly validates
user-supplied flags. The connect, ai, check, and
k8s install commands drop their private regexes
and delegate to ValidVersion.

The parser now accepts any prerelease or build
suffix, so upgrade can parse any version that
install accepts. The ai, check, and k8s
validators previously capped segments at two
digits and accepted trailing garbage; both bugs
are gone with the shared pattern.