Update Gradle to 8.1 and KMP to 1.7.21

[jsoizo]

related to: #145

I want to bump up the gradle version to 8.1 or later in order to publish to the repository via the Maven Central Portal, and then install a publish plugin that supports publishing via the Portal.

To do this, bump up the gradle version, and then bump up the KMP version as well, since the KMP plugin will not work if the gradle version is raised.

The backward compatibility issue with the increased KMP version was verified using Kotlin/binary-compatibility-validator vesrion 1.4.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.66%. Comparing base (4e0b4ab) to head (6d15cd2).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #146      +/-   ##
============================================
- Coverage     86.78%   86.66%   -0.12%     
- Complexity      164      172       +8     
============================================
  Files            21       21              
  Lines          1286     1282       -4     
  Branches        194      192       -2     
============================================
- Hits           1116     1111       -5     
- Misses           31       32       +1     
  Partials        139      139              

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