Use 3.3 instead of ruby for ruby versions

[m-nakamura145]

The 'ruby' version currently refers to version 3.3.x, but when the new 3.4.0 is released, our current setup will stop testing against 3.3.x.

Therefore, I suggest we explicitly specify version 3.3 and, upon the release of 3.4.0, add and test against version 3.4 as well.

ref: #225 (comment)

[rgrove]

Thanks, but the reason for including ruby in the test matrix is to ensure that we always test against the latest release, even if we forget to update the matrix. We can add 3.3 to the matrix when 3.4 is released, which likely won't be soon given that 3.3.0 only came out a few weeks ago.