Support DROP INDEX CONCURRENTLY

[eypcnckr]

Closes #3901

What

CREATE INDEX CONCURRENTLY has been supported since #968, but the matching drop
was not - Generate(DropIndexOperation, ...) always emitted a plain DROP INDEX
inside the migration transaction. Removing an index created with
IsCreatedConcurrently() therefore takes an ACCESS EXCLUSIVE lock on the table.

Two things were missing, and both are needed for the feature to work end to end:

• NpgsqlMigrationsAnnotationProvider did not override ForRemove(ITableIndex), so
  Npgsql:CreatedConcurrently never reached the DropIndexOperation produced by the
  differ - the operation had no annotations at all.
• Generate(DropIndexOperation, ...) did not look at the annotation.

This PR honours the annotation on both sides and suppresses the transaction for the
drop, mirroring Generate(CreateIndexOperation, ...).

Design note

I reused the existing Npgsql:CreatedConcurrently annotation instead of introducing
a new one, so IsCreatedConcurrently() now means "created and dropped
concurrently". No new public API, and I think it matches the intent - but it is a
behaviour change for existing users: dropping such an index now runs outside the
migration transaction. If you would rather have an explicit IsDroppedConcurrently(),
say the word and I will rework it; the generator half stays the same either way.

Testing

• DropIndexOperation_concurrently / DropIndexOperation_not_concurrently in
  NpgsqlMigrationsSqlGeneratorTest cover the generated SQL.
• Drop_index_concurrently in MigrationsNpgsqlTest covers the full model-diff
  path, mirroring the existing Create_index_concurrently.

What I could and could not run locally: main targets net11.0 and pins the
.NET 11 preview SDK in global.json, which I do not have installed, so I could not
build main or run its test suite. To verify the change I:

1. Applied the generator change to a hotfix/10.0.4 worktree (identical code there,
   targets net10.0), added the two NpgsqlMigrationsSqlGeneratorTest tests and ran
   them - both pass.
2. Verified the ForRemove(ITableIndex) half separately against the shipped 10.0.0
   package by registering an IMigrationsAnnotationProvider with exactly this
   override and running the migrations differ: the DropIndexOperation goes from
   having no annotations to carrying Npgsql:CreatedConcurrently = True.

So both halves are verified, but not together on main - the new
Drop_index_concurrently functional test in particular has not been executed. Please
let CI confirm, and I am happy to fix anything it turns up.

[roji]

Yeah, makes sense for me to reuse IsCreateConcurrently for deletion as well rather than introduce yet another API point - the same reasons that lead to wanting an index to be created concurrently like lead to the same thing for drop.

[eypcnckr]

@roji hi, CI failed, but the only failure is build (windows-2022, 17, Release), and it's in the Start PostgreSQL 17 (Windows) step — the job exits after ~1.5 min, before the SDK setup and build steps run. All Linux jobs (PG 13–18, Release + Debug) passed, so the new tests do pass.

Looks unrelated to the PR — could you re-run that one job when you get a chance?