gitk: Make TclTk 8.6 the minimum, allow 8.7

gitk: Make TclTk 8.6 the minimum, allow 8.7

gitk runs under wish so naturally has Tcl and Tk available and of the
same version. gitk sets a requirement on Tk version >= 8.4: this is very
outdated, and the earliest Tcl currently shipping on any supported OS is
8.6. As 8.7 is in alpha test and is generally compatible with 8.6, we
should allow 8.7. Tcl 9.0 has planned compatibility breaking changes so
is not yet supported.

Let's change the requirements to 8.6-8.7, but not 9.0. Place this at the
top of file so the requirements are obvious.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>