Make SemVersion public

[flobernd]

Follow up of #97

This is a predecessor PR for consolidating SemVersion with ElasticVersion (https://github.com/elastic/apm-agent-dotnet/blob/main/src/Elastic.Apm/ServerInfo/ElasticVersion.cs#L243).

cc @Mpdreamz @stevejgordon I'm open to naming discussions (SemVersion vs ElasticVersion), but I think SemVersion better describes what this class does.

[Mpdreamz]

Given we no longer have our 'weird' custom ToString implementation that normalizes all prereleases to p without a dash renaming to SemVersion seems appropiate.

That said since we want to publicly ship this (so we can potentially reuse this), maybe ElasticVersion remains the more appropriate?

No hard opinion on my end will accept either way!

[flobernd]

@Mpdreamz I will keep SemVersion for the new class as it models a pure semantic version without any Elasticsearch specific changes.

The Elastic.Transport/Requests/MetaData/VersionInfo.cs is still Elasticsearch specific. Should we change the name to ElasticVersionInfo?

The decision about using the full semantic version in the request headers for the Serverless clients got reverted. Currently, the semver metadata suffix encodes the API version of the serverless server which means that it's redundant information. The consensus was to stick with the p suffix for this specific usecase. However, it's still nice that we expose the metadata part in VersionInfo for this: https://github.com/elastic/elasticsearch-net/pull/7992/files#diff-b8fcb1d512c1ce688cec6e911dd56161eeaa33977a698a69b6d82cbab7f6e676R82