Bluetooth: Host: Deprecate `BT_BUF_ACL_RX_COUNT` symbol

Because the number of ACL RX buffers must be at least the number of
maximum connections plus one, increasing `CONFIG_BT_MAX_CONN` could
inadvertently lead to a build failure if the number of ACL RX buffers is
not also increased. This dependency may not be obvious to users.

To address this issue, this commit deprecates the
`CONFIG_BT_BUF_RX_COUNT` Kconfig symbol and computes the value in
`buf.h` using the new `BT_BUF_RX_COUNT` define. Note that the default
value and the minimum range value have been changed to 0 to "disable"
the option.

Additionally, to allow users to increase the number of ACL RX buffers,
this commit introduces the new `CONFIG_BT_BUF_RX_COUNT_EXTRA` Kconfig
symbol. The value of this symbol will be added to the computed value of
`BT_BUF_RX_COUNT`.

The configurations of tests and samples have been updated to reflect
these changes.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>