Fix changelog template splitting multi-line entries into duplicate bullets (#69454)

[dwoz]

What does this PR do?

Fixes the towncrier-based changelog generator, which has been splitting every multi-line
changelog fragment into one bullet per source line and appending the [#NNNN] issue link
to every single bullet. The template is replaced with a single-bullet render that uses
Jinja's indent(2) filter so wrapped fragments stay one bullet with the link appended
once. Already-published broken entries in CHANGELOG.md, pkg/rpm/salt.spec and
pkg/debian/changelog are also cleaned up.

What issues does this PR fix or reference?

Fixes #69454

Previous Behavior

A fragment such as changelog/69031.fixed.md, wrapped at ~72 columns, rendered in
CHANGELOG.md as:

- Fixed two distinct bugs in the `salt.engines.redis_sentinel` engine that [#69031](https://github.com/saltstack/salt/issues/69031)
- together prevented it from being usable. `start()` no longer raises [#69031](https://github.com/saltstack/salt/issues/69031)
- `AttributeError: 'dict_values' object has no attribute 'pop'` on Python 3 [#69031](https://github.com/saltstack/salt/issues/69031)
...

Eight bullets, each with the same duplicated [#69031] link, sentences sheared in half
at the source line breaks. Repeated for every multi-line fragment in the
v3008.0/v3008.1 release notes and across pkg/rpm/salt.spec and pkg/debian/changelog.

New Behavior

- Fixed two distinct bugs in the `salt.engines.redis_sentinel` engine that
  together prevented it from being usable. `start()` no longer raises
  `AttributeError: 'dict_values' object has no attribute 'pop'` on Python 3
  ...
  working unchanged. [#69031](https://github.com/saltstack/salt/issues/69031)

One top-level bullet, continuation lines indented under it (Markdown reads them as part
of the same list item), and the issue link appended exactly once at the end.

Fragments that intentionally include Markdown-style nested sub-bullets ( - item) are
unaffected: their indentation is preserved verbatim and they continue to render as a
nested sub-list under the single top-level bullet.

Scope of the diff

1. changelog/.template.jinja — replace the per-line loop introduced in
   808241d3154d with a single-bullet indent(2) render.
2. tests/pytests/unit/changelog/test_template.py — new TDD-verified unit
   tests covering single-line, multi-line wrapped, multi-fragment, and
   nested-sub-bullet cases. The multi-line test fails on the unmodified
   template (8 top-level bullets where 1 is expected) and passes with the
   fix.
3. CHANGELOG.md, pkg/rpm/salt.spec, pkg/debian/changelog — 118
   already-published broken multi-bullet entries consolidated back into
   single bullets. Original sentence content is preserved verbatim; only
   the broken line breaks between adjacent same-link bullets are stitched
   back together. Affected release sections: 3008.1, 3008.0,
   3008.0rc4, 3008.0rc3, 3008.0rc2, 3008.0rc1, 3006.25.

Merge requirements satisfied?

• Docs (no documented behavior change; only the rendered output of an internal tool)
• Changelog (changelog/69454.fixed.md)
• Tests written/updated (tests/pytests/unit/changelog/test_template.py)

Commits signed with GPG?

Yes