[BUG] Remove references to the deprecated v8 and amsterdam of OUI

[AMoo-Miki]

Describe the bug

OUI will be removing the deprecated theme files in their next minor release which would break builds.

[Nicksqain]

Do we need to remove this all in 2.6 ( for 2.7 in next?)

[joshuarrrr]

As with most PRs, we'll start with PRs against main, and maintainers will then backport to active release branches as necessary.

[joshuarrrr]

Removing amsterdam references should be relatively straightforward: https://github.com/search?q=repo%3Aopensearch-project%2FOpenSearch-Dashboards%20amsterdam&type=code

Finding v8 theme references will need a bit more care, because there's also plenty of comments and codes that use v8 in other contexts (JS engines, node, etc.).