Rename Tested Versions header

What does this MR do and why?

While working on a doc update Remove minor from supported pnpm lockfile versions (!130673 - merged) I noticed that the Tested Versions column title was a bit confusing, since it doesn't explain what version is being tested - the lockfile or the package manager? This MR makes the name of the title more explicit by using the longer name Tested Package Manager Versions.

This was was previously discussed here and we decided to use Tested Versions instead of Tested Package Manager Versions because it was shorter, however, since we have no shortage of space in the table, we might as well use the more descriptive name, which this MR implements.

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