** Implementation details **

We disable DDL by patching the existing on_replace_dd_system_space
trigger callback installed for each system space so that now it raises
an error in case the current schema version is less than the most
recent one known to this build. Since to perform a schema upgrade
we need to execute DDL, we suppress the error for the fiber that is
currently running a schema upgrade. To achieve that, the upgrade script
calls box_schema_upgrade_begin and box_schema_upgrade_end before
starting and after completing a schema upgrade. The functions keep track
of the fiber that is currently running a schema upgrade so that we can
allow all DDL operations for it. We also allow DDL during recovery so
that we can replay DDL statements written to the WAL.

Since there may be a bug in the `box.schema.upgrade` implementation,
we export `box.internal.run_schema_upgrade`, which runs the given
function as a schema upgrade script (allowing DDL). The user may use
this function to recover after a schema upgrade failure.

** Note about the tests **

A test server instance started by luatest grants permissions to the
guest user so that luatest can execute commands on it. It means that if
a test uses a generated snap file committed to the repository for a test
server instance, it will fail because granting permissions is a DDL
operation. To prevent this, we have to regenerate snap files so that
they contain all required permissions. This works because a test server
instance grants permissions with the `if_not_exists` flag.

The problem is that it isn't easy to regenerate the snap files for the
following tests because there's no generator script:
 - `test/box-luatest/gh_6794_recover_nonmatching_xlogs_test.lua`
 - `test/box-luatest/gh_7974_force_recovery_bugs_test.lua`

So we temporarily disable these tests and file tickets to fix them.

Other notes:
 - We drop `test/box-luatest/upgrade/2.9.1` and make the test using it
   use `test/box-luatest/upgrade/2.10.0` instead. We do this because
   2.9.1 was never released and the earliest Tarantool version using the
   2.9.1 schema version is 2.10.0. This shouldn't affect the test
   anyhow.
 - We drop the part of the `user_auth_history_last_modified_upgrade`
   test that checks that creating users/roles with an old schema works
   fine because this is forbidden now.
 - We wrap the code that creates a space with an old schema in the
   downgrade test in `box.internal.run_schema_upgrade`. Even though it's
   unsupported now, we still need to check that space creation works
   after a downgrade.

Closes tarantool#7149

@TarantoolBot document
Title: Document that DDL is disabled with an old system schema

Executing DDL operations with an old (not upgraded) system schema is
dangerous and might result in unexpected breakages. So we decided to
explicitly forbid all DDL operations with an old system schema until
`box.schema.upgrade()` is called.  Note, one can still call `box.schema`
functions with an old schema provided they do nothing, for example, if
an object is created with the `if_not_exists` flag and the object with
same id already exists:

```lua
box.schema.create_space('test', {if_not_exists = true})
```

Otherwise an attempt to create a space with an old schema will raise
an error like shown below:

```yaml
tarantool> box.schema.space.create('test')
---
- error: Your schema version is 1.6.8 while Tarantool
    3.0.0-entrypoint-262-g3eaba1cef686 requires a more recent
    schema version. Please, consider using box.schema.upgrade().
...
```