This commit adds a script to sync the version present in .go-version
across all go.mod files as the toolchain directive. As part of that,
this commit also modifies go.mod files that did not have synced toolchain
directives.
Additionally, this also adds a script to verify all toolchain and go
directives against the version present in .go-version as follows:
(1) The go directive <= version in .go-version
(2) The toolchain directive == version in .go-version
This script runs as part of the `make verify` target, making it run
as a presbumit by default.
Signed-off-by: Madhav Jivrajani <madhav.jiv@gmail.com>
