read-only for lc_messages breaks compatible with phpmyadmin

[Defined2014]

Bug Report

see details from #33708 (comment)

[morgo]

There are a few ways you could fix this:

1. Change it back to a noop
2. Protect it by noop-functions
3. Instead of read-only, write a validation func so that it only errors when trying to set it to something other than en_US.
4. Same as (3) but instead of an error return a warning and refuse changes to other than en_US.

I don't love noops (they are confusing to debug), but I think (4) is a safe enough tradeoff for this variable.

@williamdes @kvtb what do you think?

[williamdes]

I would vote for Change it back to a noop so no apps should be broken by that behavior

[djshow832]

I agree with Change it back to a noop. Protecting it by noop-functions also needs the user to set tidb_enable_noop_functions to true. Considering the user upgrades TiDB, it breaks compatibility.

[Defined2014]

Agree with Change it back to a noop, so maybe we could revert #33708