…height (#2112)

Closes #2109

In `v0.38.2`, when the App was setting `VoteExtensionsEnableHeight` to a
value that was the same as the current one, even if is useless as a
ConsensusParams update, we were accepting it because it doesn't do any
harm.

When we released `v0.38.3` we refactored function `ValidateUpdate` and
we started rejecting those (useless but harmless) cases.

Since `v0.38.3` is a point release, we shouldn't add restrictions on
inputs to a function without a good reason for it. This PR is adding a
rule to `ValidateUpdate` so re-allow all cases where the app is not
really updating the value of `VoteExtensionsEnableHeight`, regardless of
which height we are currently in.

### Testing done

Apart form CI tasks, we carried out the following extra (manual)
activities:
* We reproduced the error reported (#2109) using our e2e test infra, by
configuring the manifest to set a vote extension to a height in the
past, which is the same height that was previously passed via the
genesis file
* [THE FIX] In the production code, we introduced an early rule in
function `ValidateUpdate` to return no error if the value to update is
equal to the current value
* We also updated two unit tests that should not be returning an error
now (and that were returning an error in `v0.38.3`)
* We re-ran the e2e tests as explained above with the new fix, and the
problem reported in #2109 no longer shows up
* Then, to make sure no other extra restrictions were introduced in
`v0.38.3`, we did the following:
* We temporarily replaced the implementation of `ValidateUpdate` with
the one existing before `v0.38.3` (these temporary changes are not
included in the PR, it was just done for testing pruposes)
* We ran the new exhaustive unit tests introduced in `v0.38.3` and
modified in this PR against the replaced (`v0.38.3`) implementation
* We went through all the UTs that are now failing in
`TestConsensusParamsUpdate_VoteExtensionsEnableHeight` (getting an
unexpected error, or not getting an expected error), to make sure they
don't represent a set of inputs to `ValidateUpdate` that were accepted
in v0.38.2 but no longer accepted with the current version (the one in
this PR)
  * We found no problematic test case

---

#### PR checklist

- [x] Tests written/updated
- [ ] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

---------

Co-authored-by: Greg Szabo <16846635+greg-szabo@users.noreply.github.com>
Co-authored-by: Greg Szabo <greg@philosobear.com>