This answer disagrees that major changes to the migration tag excerpt or wiki are required.
Maybe the existing content could be tweaked slightly to emphasise that the tag is primarily for one-off or irregular data migrations (discussed below).
It seems wrong (to take an example) to tag a question about moving data from one physical host or location to another (on the same platform and version) as upgrade, etl, installation, or configuration. None of those tags have suitable excerpts for this example:
Upgrade implies a (usually upward) change to version or release on the same platform. (It is fairly common to hear side-by-side (not in-place) upgrades referred to as migrations, but I believe these are more usefully tagged as upgrades). If the software isn't changing, it isn't an upgrade.
ETL (or ELT) is certainly related to data migration, but it is tough to argue that one is a proper superset of the other. ETL is most commonly associated with regular, similar, data transfer operations, usually including transformation, and often associated with maintaining a derived or aggregated view of the data e.g. in a data warehouse.
A migration is most usually a large, one-off, or irregular exercise. ETL may be one means to achieve a migration (in some cases), but it is hardly the most common one. I wouldn't personally go looking for such questions under etl, and wouldn't expect site ETL tag statistics to include data migrations.
The argument that installation or configuration somehow necessarily includes any data transfer seems a very weak one to me (and quite counter-intuitive). People searching or subscribing to these tags expect to find questions matching their excerpts: installing a product or application, and choosing settings respectively. Nowhere do they say anything about moving data around, and nor should they, in my view.
Attempting to enforce some particular interpretation of the word "migration" may (or may not) be more technically correct, but if all it does it add to the site's housekeeping workload without any clear, practical benefits, it is a bad change. Especially so without a team of people willing to commit to maintain that interpretation and its application over the long term.
Anyone looking for question tag maintenance work to do on the site will surely not be short of higher-value things to do already.