Remove duplicate semantic versioning types

[anferbui]

Description

In the codebase, we have four types which we use to represent a semantic version number:

• SemanticVersion which looks like a copy of SymbolKit.SymbolGraph.SemanticVersion
• Version
• VersionTriplet
• Platform.Version

These all seem to be fulfilling a similar purpose. We should converge on using only one type throughout the DocC codebase.

Checklist

• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue.

Expected Behavior

Only one Swift type is used to represent a semantic version number in DocC. All other types are deprecated/removed.

Actual behavior

There are 4 types used to represent a semantic version number in DocC.

Steps To Reproduce

No response

Swift-DocC Version Information

f019ab8

Swift Compiler Version Information

No response

[anferbui]

Per discussion in #959, @d-ronnqvist and me think we should try to move towards converging on SemanticVersion because it's the type that is used the most in the repo.