github: move GitHub issue template to .github directory

ISSUE_TEMPLATE.md is file specific to GitHub issue reporting
feature. For some time, .github directory is one that
can be used for all related files.

This patch just moves the template to .github directory.
Further patches will add more functionalities in this folder.

Signed-off-by: Tomasz Zawadzki <[email protected]>
Change-Id: I3e1e06dd152f489ab178bf7eac74432fd5b61de4
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1597
Tested-by: SPDK CI Jenkins <[email protected]>
Reviewed-by: Jim Harris <[email protected]>
Reviewed-by: Karol Latecki <[email protected]>
Reviewed-by: Ben Walker <[email protected]>
Reviewed-by: Darek Stojaczyk <[email protected]>