[chore] Different error message for email validation from net/mail parsing on go 1.21.8 and above

[blakesmith]

go 1.21.8 fixed some minor issues in net/mail that causes the test suite to fail for some mail validation cases. Although we're not on go 1.21.8 yet, make the test forward and backwards compatible.

See: golang/go@263c059

Running the test suite with go 1.21.8 leads to test failures like:

-- FAIL: TestValidationTestSuite (0.00s)
    --- FAIL: TestValidationTestSuite/TestValidateEmail (0.00s)
        formvalidation_test.go:147:
                Error Trace:    /home/blake/src/gotosocial/internal/validate/formvalidation_test.go:147
                Error:          Not equal:
                                expected: &errors.errorString{s:"mail: no angle-addr"}
                                actual  : &errors.errorString{s:"mail: missing word in phrase: mail: invalid string"}

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,3 +1,3 @@
                                 (*errors.errorString)({
                                - s: (string) (len=19) "mail: no angle-addr"
                                + s: (string) (len=50) "mail: missing word in phrase: mail: invalid string"
                                 })
                Test:           TestValidationTestSuite/TestValidateEmail
FAIL
FAIL    github.com/superseriousbusiness/gotosocial/internal/validate    0.006s

This cropped up when trying to update the gotosocial nix package to 0.14.2, while at the same time nix updated its go release to 1.21.8. NixOS/nixpkgs#295584

Checklist

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[tsmethurst]

Looks good, thanks for the fix :)