docs: Fix changelog entries (#1166)

Fixes to `CHANGELOG.md`:

- 3.4.0: decode `/v2` to `/v2`.
- 3.4.0: drop the "Allow impit v0.15" (#1128) entry. Dropping the impit
upper bound (#1163) covers it.
- Decode the remaining HTML entities (`'`, `>`, `"`,
`/`) in older releases.
- 2.5.1: link #802 and add its author.
- 3.2.0: fix the broken `renovate[bot]` author link to match the 3.0.4
one.
- Capitalize "Actor" in three entries and "Drop" in the 1.8.0 note, and
remove a double space.
- Use two blank lines between all releases.
- 1.6.2 and 1.6.3: fix the release year (2024, not 2023).

*✍️ Drafted by Claude Code*

Author: vdusek