Micro vias are too restricted

Description

Microvias can only be placed on the outer layers (F.Cu/B.Cu) and the ones directly adjacent to them.

This is not true. Microvias are placed at the outside, yes, but not exclusively as a final manufacturing step. I'm currently working on a project whose stackup has microvias on the outside of an inner 6-layer core, then 2 more layers stacked on top, then microvias on those as well. So microvias are possible on L1-L2, L2-L3, L1-L3, L6-L7, L7-L8, and L6-L8. The outer-layers-only restriction forces me to give up the convenience of a different default via size for micros, which is really a necessary "convenience" on a dense, complicated board with a ton of vias.

This stackup has B/B mechanical drills ALSO, so just using B/B doesn't really get me what I need unless I'm willing to give up some validation of hole sizes and select the sizes manually (ick)

I think we should remove this restriction, as well as adding a "Select Layer and Place Microvia" companion to "Select Layer and Place Blind/Buried Via".

Steps to reproduce

1. Try to place a microvia while on layer 6 out of 8.
2. Get yelled at

KiCad Version

Application: KiCad PCB Editor

Version: 5.99.0-unknown-11110+g6d223931fb+buster, release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.64.0 OpenSSL/1.1.1d zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.5) libssh2/1.8.0 nghttp2/1.36.0 librtmp/2.3

Platform: Linux 5.9.0-0.bpo.5-amd64 x86_64, 64 bit, Little endian, wxGTK, xfce, tty

Build Info:
	Date: Jun 21 2021 19:13:58
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.67.0
	OCC: 7.3.0
	Curl: 7.64.0
	ngspice: 30
	Compiler: GCC 8.3.0 with C++ ABI 1013

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON