Update changelogs with releases 1.24.1 through 1.26.0rc3

[kindermax]

What changed

• CHANGELOG.md: added entries for every release since 1.24.0, reconstructed from git tags and commit history:
  • Unreleased — gRPC server, protobuf definitions and related dependencies removed (Remove gRPC server, protobuf definitions and related dependencies #68)
  • 1.26.0rc3 — flag_state Prometheus gauge with flag/project labels
  • 1.26.0rc2 — env-var substitution in config ($VAR indirection for OIDC client_secret, ${VAR} and inline substitution)
  • 1.26.0rc1 — OIDC authentication support
  • 1.25.0 — project label on handler metrics, Prometheus metrics docs
  • 1.24.1–1.24.4 — RST docs setup, UI conditions ordering/layout, GraphiQL fix, value condition override input fixes
• docs/changelog/server/changes_1.rst: mirrored the same entries in the Sphinx docs changelog, which also stopped at 1.24.0
• docs/changelog/server/index.rst: dropped the dangling changes_2 toctree reference — c4ec1da added it ("add 1.25 changelog") but the file was never committed, leaving a broken reference in the docs build

Why

The changelogs were five releases behind; the docs changelog additionally had a broken toctree entry.

Notes for reviewers

• New entries were folded into changes_1.rst ("Changes in 1.X") rather than creating changes_2.rst, since all releases are still 1.x.
• Verified with pdm run docs (sphinx-build): build succeeds with zero warnings or errors.
• Release dates are taken from tag commit dates; entry contents summarize the commit messages between tags.