devel-docs: for the 3.0 series, Win 10 becomes our minimal support.

After discussions on IRC with ender/Jernej and frogonia:

* exchndl.dll (drmingw) requires dbgcore.dll and dbghelp.dll which seem to ship
  with win10+, but not win7 (we don't know for Win8). So for Win7, Jernej had to
  manually add it back (otherwise, building then running on Win10, it looks
  fine, but GIMP fails to start on Win7).
* MSYS2 dropped Win7 support anyway and supports now Win 8.1+.
  See: https://www.msys2.org/docs/windows_support/
  So even though it looks like GIMP works fine on Win7 (after special-casing the
  2 DLLs above by adding them manually in the installed files), there might be
  unforeseen issues in the long run.
* Win7 official support by Microsoft now ended. And Win8 support is soon to be
  (it actually already has, "except for Server 2012 [based on Win8], which's
  supported for another 2 months"). Win8 never had much adoption anyway.
* frogonia confirms they didn't see many Win 7 reports, if at all, in the last
  year.

So based on all these, and in order not to make our life over-complicated, we
drop Win 7/8 support for the next 2.99 versions and for the upcoming stable 3.0
series.

Though we keep support to these versions in the 2.10 series, in order not to
disrupt current usage too much.

devel-docs/os-support.txt

@@ -18,9 +18,15 @@ After GIMP 3.0 release, we might get back to depend on Debian Testing.
 ### macOS
 
 Compatibility with 10.13 and over.
-64-bit only.
+
+Hardware:
+* x86_64 (Intel)
+* ARM 64-bit (Apple Silicon)
 
 ### Windows
 
-Windows 7 and over.
-32 and 64-bit.
+Windows 10 and over.
+
+Hardware:
+* x86 32 and 64-bit
+* ARM 64-bit (experimental)