doc: add changelogs for the upcoming 2.8.1 release

See https://github.com/tarantool/tarantool/discussions/5634

changelogs/unreleased/cmake-3.md

+## feature/build
+
+* Updated CMake minimum required version in Tarantool build infrastructure to 3.1.

changelogs/unreleased/drop-autotools.md

+## feature/build
+
+* Drop autotools dependencies from RPM / Deb packages (follows up gh-4968).

changelogs/unreleased/drop-centos-6.md

+## feature/build
+
+* Stop publishing new binary packages for CentOS 6.

changelogs/unreleased/drop-debian-jessie.md

+## feature/build
+
+* Stop publishing new binary packages for Debian Jessie.

changelogs/unreleased/drop-mac-os-10.13.md

+## feature/build
+
+* Stop support of Mac OS 10.13 (disable regular testing).

changelogs/unreleased/exclude-null.md

+## feature/core
+
+* Introduce exclude_null option to index part definition. When it is on, index
+  filters and doesn't store tuples with "null" value of this part (gh-4480).

changelogs/unreleased/fedora-33.md

+## feature/build
+
+* Publish binary packages for Fedora 33 (gh-5502).

changelogs/unreleased/fix-freebsd-build.md

+## bugfix/build
+
+* Fix building on FreeBSD (incomplete definition of type 'struct sockaddr')
+  (gh-5748).

changelogs/unreleased/github-actions.md

+## feature/testing
+
+* We're moving to GitHub Actions (gh-5662).

changelogs/unreleased/jepsen-in-ci.md

+## feature/testing
+
+* Run single node Jepsen testing on per-push basis (gh-5736).

changelogs/unreleased/static-build-dont-refetch.md

+## bugfix/build
+
+* Don't refetch already downloaded static build dependencies (gh-5761).

changelogs/unreleased/zstd-1.4.8.md

+## feature/build
+
+* Bump built-in zstd version from v1.3.3 to v1.4.8 (part of gh-5502).