Document `box_space_execute_priv` compatibility option

Product: Tarantool
Since: 3.0
Root document: new page in https://www.tarantool.io/en/doc/latest/reference/reference_lua/compat/;
check if changes are needed in https://www.tarantool.io/en/doc/latest/book/admin/access_control/
SME: @ locker

Details

Historically, it was possible to grant the execute privilege on
a space although this action had no effect. Since Tarantool 3.0
it isn't allowed anymore. The new compat module option
box_space_execute_priv was added to revert to the old behavior.

Please create a documentation page for the new compatibility option at
https://tarantool.io/compat/box_space_execute_priv

Example:

tarantool> box.cfg{log_level = 'error'}
---
...

tarantool> box.schema.user.create('alice')
---
...

tarantool> box.schema.user.grant('alice', 'execute', 'space')
---
- error: Unsupported space privilege 'execute'
...

tarantool> require('compat').box_space_execute_priv = 'old'
---
...

tarantool> box.schema.user.grant('alice', 'execute', 'space')
---
...

Requested by @locker in https://github.com/tarantool/tarantool/commit/21063b4ae92cb8adc8dbe89bdf7fe028fc2d2ac9.

Definition of done

• a page about compat.box_space_execute_priv is added to the compat module reference
• the compat.box_space_execute_priv description in the configuration reference has a "see also" link to the option reference page
• execute privilege is never applied to spaces in docs

Start with the compatibility option reference at https://www.tarantool.io/en/doc/latest/reference/reference_lua/compat/ and review the configuration reference for related access-control text. Add the compat.box_space_execute_priv page, link to it from the configuration description, and ensure the documentation does not apply execute privileges to spaces.