Formally deprecate -blocks-storage.tsdb.max-exemplars

[CharlieTLe]

Groundwork for v1.22.0. Refs #5922. Optional — happy to drop this if the maintainers would
rather not start another clock this cycle.

-blocks-storage.tsdb.max-exemplars has called itself deprecated in its flag help for a
long time:

"Deprecated, use maxExemplars in limits instead. ..."

But the deprecation was never announced in CHANGELOG.md, so the two-minor removal
window in v1-guarantees.md never actually started, and there is no signal an operator can
alert on.

It is also still live code, not an overdue no-op. Ingester.getMaxExemplars uses it as
the global fallback whenever a tenant's max_exemplars limit is 0:

if maxExemplarsFromLimits == 0 {
    return int64(i.cfg.BlocksStorageConfig.TSDB.MaxExemplars)
}

Removing it now would silently disable exemplars for every tenant relying on that fallback.

What this does instead

Starts the clock properly. The flag keeps working exactly as before, but:

• the help text now names v1.24.0 as the removal target and describes what it actually
  does, rather than just "use maxExemplars in limits instead";
• setting it to a non-zero value logs a warning at ingester startup and increments
  deprecated_flags_inuse_total, so operators can find and migrate the usages — which is
  the whole point of a formal deprecation.

Removal in 1.24.0, two minors from now, per v1-guarantees.md.

Verification

go build ./..., make doc (generated config reference and JSON schema regenerated), and
./pkg/ingester/ ./pkg/storage/tsdb/ green.