10.x: reject a collation on a string set column unless it compares bytes (port of #483) #496

Problem

On main, once #483 merges (PR #492), a collation on a StringSet/StringSet<T> column that may not compare bytes is rejected when the model is built, and so is EF.Functions.Collate over a string set. On release/10.x, a case-insensitive collation still makes Contains, the subset operators and == answer differently from memory, and ExecuteUpdate with Remove("a") deletes A.

Change

Cherry-pick PR #492's commits with -x into a PR against release/10.x:

• StringSetCollation (the convention and its IConventionSetPlugin), its registration, and the guard's EF.Functions.Collate refusal.
• StringSetCollationTests and StringSetCollationIntegrationTests.
• The EF guide's Good to know entry, claim E86.
• 10.x specifics: the changelog row goes under ### Changed answers in [10.4.0], linking Text value set operators follow a nondeterministic column collation: on a case-insensitive column, Contains("a") is true for {A} and Remove("a") deletes A #483 and its migration section. docs/migration.md gets ## Changed answers in 10.4.0 listing Check string set columns with a collation. pr-hygiene.sh requires both for a silent PR into a minor.

Done when

On release/10.x, the tests pass on the EF Core 10.0.0 floor and on CI's newest-patch leg, and both halves fail with their fix reverted.

Release

### Changed answers in 10.4.0. Fixed only on release/10.x, so close it by hand.