`TokenService` clean up

[n1v0lg]

The TokenService could use some spring cleaning 🧹 🧹 🧹

It has evolved quite organically; a sizeable portion of the code is either (1) not actually used anymore or (2) there for backwards compatibility with older ES versions (pre-7.17).

This issue tracks the various cuts we can make.

• Remove dead code (several areas, but here is an example)
• Remove key rotation code (e.g. this). This code was never actually used.
• TokenService - drop BWC support for tokens created in pre-7.17 ES versions  #87726

[elasticmachine]

Pinging @elastic/es-security (Team:Security)