Give returning v5 users the assertion rename map

[nohwnd]

Fix #463

Last of the stack, on top of #467. Review #464, #465, #466, #467 first.

The tutorial says Should-Be and Should -Be both work, which is true, I mixed them inside one It and both passed. Then it uses Should-HaveType, Should-BeCollection, Should-BeNull and Should-BeLessThan without saying which v5 operator each one replaces. The names are not a mechanical translation of the old parameters, so guessing fails with a plain CommandNotFoundException and no suggestion. I guessed eight and got eight of those.

Added a table of the assertions the tutorial uses plus the few that catch people out, and a line saying the old operators are not deprecated so leaving a suite alone is a valid choice.

Three related things.

tutorial/5-working-with-files/2-testdrive.mdx uses Test-Path piped into Should-BeTrue and reads like a style choice. There is no Should-Exist in the new family, so it is the replacement, and now the page says that.

tutorial/4-mocking/3-verifying-calls.mdx teaches Should-Invoke from scratch and never says where Assert-MockCalled went. Same page framed the mock fall-through removal as a nice property of a new suite, when it is the change most likely to turn an existing green v5 suite red. Both now say so.

docs/migrations/v5-to-v6.mdx documents the Assert-MockCalled symptom as:

The term 'Assert-MockCalled' is not recognized as a name of a cmdlet, function, script file, or operable program.

That is not what you get. The command is still listed in the manifest, so PowerShell tries to auto-load Pester and reports the load failure instead:

The 'Assert-MockCalled' command was found in the module 'Pester', but the module could not be
loaded due to the following error: [Should operator 'Be' is not registered]

Somebody searching for the message they actually saw does not find that section, which is the one job the Symptom block has.

Verification

Every row of the table is a passing test rather than something I remembered, each one asserting the v5 form and the v6 form against the same value:

Describing v5 to v6 assertion mapping
Tests Passed: 8, Failed: 0, Skipped: 0, Inconclusive: 0, NotRun: 0

That includes confirming Should-Exist does not exist. The Assert-MockCalled message above is copied from a real 6.1.0 session. Site builds.

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