Many of our scripts have a non-portable interpreter line for bash and
use bash-specific variables like `BASH_SOURCE`. Update the interpreter
line to be portable between various Linuxes and macOS without
complaint from posix shell users.

This changeset fixes two sources of flakiness in the event stream test.

First, the stream request gets the event *closest* to the index, not
the exact match. Although events are written before raft entries
they're written asynchronously, so it's possible to race and get a
raft index from this query higher than the current head of the event
buffer. Ensure the job is running before we try to get the index, so
that we've given the event enough time to land in the buffer.

Second, the assertion that the found index is greater than the start
index is only true if the `PlanResult` event manages to land before we
do the second registration. Although it should now with the first fix
above, it's not a correct assertion for what we're testing.

feat: remove dependency to consul/lib

Signed-off-by: Yoan Blanc <yoan@dosimple.ch>

The oversubscription test expects an output that requires the client
has polled the task for stats at least once. Wait long enough to
ensure that we've polled the stats before failing the test.

In the reconciler's filtering for tainted nodes, we use whether the
server supports disconnected clients as a gate to a bunch of our
logic, but this doesn't account for cases where the job doesn't have
`max_client_disconnect`. The only real consequence of this appears to
be that allocs on disconnected nodes are marked "complete" instead of
"lost".

fix(plugins): should return when ctx.Done

Some tests may chose to deregister jobs to check Nomad cleanup
logic, however, it is still possible for the test to fail and exit
before this is hit. This therefore adds a cancellable cleanup func
which can be deferred, using context to control whether it gets
run or not.

Signed-off-by: Yoan Blanc <yoan@dosimple.ch>

docs: fixup title formatting in upgrade guide

Add custom variable validation to docs

Custom variable validation is a useful feature that is supported by
Nomad and not just Terraform. As such it should be documented on the
input variable page.
I've cribbed the content from the terraform docs so this should be
consistent across projects

docs: add token_last_renewal and token_next_renewal to server metrics and key metrics #12435 (#12505)