Does cabal-3.10 impose `setup.Cabal < 3.12`?

When trying to make `cabal-3.10` build packages with custom setups on GHC 9.10.0 (alpha3), which ships with `Cabal-3.12`, cabal insists to build `Cabal-3.10.3.0`.
If I try to force using the installed `Cabal`, constraint solving fails.
```
system-filepath-0.4.14$ cabal build -w ghc-9.10.0 -c 'setup.Cabal >=3.12'
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: system-filepath-0.4.14 (user goal)
[__1] next goal: system-filepath:setup.Cabal (dependency of system-filepath)
[__1] rejecting: system-filepath:setup.Cabal-3.12.0.0/installed-c30b
(constraint from maximum version of Cabal used by Setup.hs requires <3.12)
```
The package itself does not impose an upper bound on `Cabal`:
```
custom-setup
setup-depends: Cabal >= 1.8, base >=4.0 && <5
```

This is not a problem with `cabal-3.11` (master), this one does not impose this upper bound.

The docs say (https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-custom-setup-setup-depends):
> If the field is not specified the implicit package set will be used. The package set contains packages bundled with GHC (i.e. base, bytestring) and specifically Cabal. The specific bounds are put on Cabal dependency: lower-bound is inferred from [cabal-version](https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-cabal-version), and the upper-bound is < 1.25.

(Ok, the "< 1.25" is definitively stale doc here.)
The docs seem to say that the upper bound comes from whatever is bundled with GHC, which should be including `Cabal-3.12` (bundled with GHC 9.10 alpha3).

Reproduce the reported command with cabal-3.10, GHC 9.10.0 alpha3, and system-filepath-0.4.14, then compare its custom-setup dependency handling with cabal-3.11. Check why Cabal-3.10.3.0 is selected despite the installed Cabal-3.12 and determine whether the implementation or the linked documentation needs correction.

Written by the indexing model from the issue text.