release: drop "-beta" suffix from 3.0.0 references - #525

Following PR amule-project#521's review discussion (see comments thread), dropping
the `-beta` suffix and shipping as plain `3.0.0`, with `3.0.x`
for follow-up fixes.  Vollstrecker's argument carried the day:
distros tend to ignore betas, so a beta cycle would route most
testing through users following git directly, which is the same
pool that's been involved across the development cycle anyway.
Plain `3.0.0` reaches the wider distro packagers immediately;
issues found post-release become `3.0.1` etc.

Three trivial edits, one mechanical theme:

 - `docs/CHANGELOG.md`: "Version 3.0.0-beta" → "Version 3.0.0"
   in the entry header; drop the "published as a beta to gather
   wider testing before declaring stable" sentence from the lead
   paragraph; rename the "Known Limitations (3.0.0-beta)" section
   header to plain "Known Limitations" and rework its lead from
   "This release is published as **beta**..." to a plain "Known
   caveats users should be aware of for this release:".  The
   bullet points themselves (macOS notarization, Flathub, AppImage
   launcher, libayatana-appindicator deprecation) stay — they're
   real limitations regardless of beta classification.

 - `.github/workflows/packaging.yml`: two illustrative comments in
   the post-build smoke-test step previously read "aMuleD
   3.0.0-beta ..." as their tagged-release example; updated to
   "aMuleD 3.0.0 ...".  The grep pattern (`'^aMuleD '`) accepts
   any tagged release shape unchanged; this is comment-only.