Remove as many getters and setters as possible from the Project class, in favor of public, typed properties

Problem/Motivation

The \Drupal\project_browser\ProjectBrowser\Project class comprises an army of getters and setters that really just access private properties.

Now that PHP supports typed, readonly properties, I'm not sure we really need this stuff. I would suggest we replace as many getters and setters as possible with public, typed properties. We could even use constructor property promotion, which would allow calls like this:

new Project(
  isCompatible: true,
  isMaintained: true,
  isActive: true,
  projectUsageTotal: 39,
);

...lots of flexibility is available to us. And we can remove a lot of boilerplate.

The outstanding questions:

• Should some (or all) of these properties also be readonly? Is there any reason to expect that code will want to -- or even should -- mutate these properties? ANSWER: There may or may not be reason to mutate them, but let's keep them mutable for now to keep the current behavior intact. Making them read-only has backwards compatibility implications.
• Should any of the properties remain private on purpose, mediated by getters and setters? I'd imagine that such a thing would only be necessary for properties which need a little extra special handling. ANSWER: Only setSummary() needed this treatment, since it has actual logic. The other properties didn't, so they are good candidates for just becoming public.