restricting Flint version should be conditional on installing Singular

[dimpase]

Steps To Reproduce

the current 10.4.beta3 does not allow external Singular, because the external flint is too new. #37495 is placing an unconditional upper bound on flint's version, for no good reason.

Users complain, rightly: https://groups.google.com/g/sage-devel/c/7RKSyswB2VA/m/T-xbNnW5BAAJ
Developers waste time, too.

Expected Behavior

External Singular should be allowed even even Flint's version is too new to build Sage's Singular, these are different things.

Actual Behavior

external Singular rejected due to Flint too new.

Additional Information

No response

Environment

- **OS**: Ubuntu 22
- **Sage Version**: 10.4.beta3

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[dimpase]

@mkoeppe @vbraun @orlitzky @antonio-rojas