Discouraged from giving A's, but students are doing excellent work, what can a professor do?

Take special care defining the rules, previously, and stick to them. You should have a specific list of objectives and the associated grades. This is implicit on tests, more explicit on projects. For instance, when elaborating written exams, I usually try to cover the whole syllabus (for that part), with questions of different difficulties, for instance, 4 questions, one easy, two moderate, one harder. That way I can easily see who really knows the content and who is just coasting by...

If a student reaches all predefined objectives, that person earned an A. And you can easily prove it by showing that he indeed reached the objectives. It would be highly unethical not to give a deserving student an A for political concerns of some suit....

There's no way to answer this based on the information given in the question.

Suppose, for example, that the course we're talking about is French 100. Students are learning how to conjugate être and so on. In this course, there is effectively a ceiling on what the students can do: they can succeed on 100% of the test questions.

But let's say instead that it's a creative writing course. A student's grade is based on short stories they wrote, and these stories are expected to show creativity, originality, and style. There is no ceiling here. It would be perfectly reasonable for the department to say that their threshold for an A in this course is very high.

There is a continuum of possibilities between these two extremes.

The problem is that there are students doing excellent work.

Doing "excellent" work doesn't necessarily make someone entitled to an A.

Retain copies of student work. Use it as a basis for a discussion about department standards versus your standards. If your expectations align with those of your department, and if you have students earning A's, then you can use the student work to defend your grades to the dean. If expectations don't align, then you can make an effort to bring them into alignment. This includes both the criteria for earning the grades (which should be clearly explained in the syllabus) and the level of difficulty of the work required (which can be somewhat subjective). It may be too late this semester, but going forward you can do change. Of course, you can in principle exercise your rights of academic freedom, which includes setting standards in your course. This may not be without consequence for yourself or the department.