fix: handle unknown alembic revisions during startup

[nhoening]

Description

Changing the branch, where a specific recent db migration from another unmerged branch is unknown, leads to this startup error:

  File "/home/nicolas/workspace/seita/flexmeasures/.venv/lib/python3.12/site-packages/alembic/script/revision.py", line 1481, in _collect_upgrade_revisions
    raise RangeNotAncestorError(lower, upper)
alembic.script.revision.RangeNotAncestorError: Revision 4b0f2e9c1a6d is not an ancestor of revision b2c3d4e5f6a7

This affects developers, mostly. This PR solves this problem

• Do not trip over unknown revision when checking if we are on HEAD at startup
• Added changelog item in documentation/changelog.rst

[read-the-docs-community]

Documentation build overview

📚 flexmeasures | 🛠️ Build #34359428 | 📁 Comparing 0958dea against latest (8944a8b)

  🔍 Preview build  

3 files changed

± changelog.html
± _autosummary/flexmeasures.utils.unit_utils.html
± api/v3_0.html

[Ahmad-Wahid]

I tried to fail test_database_schema_has_revision_false_when_current_revision_is_unknown this way.

1. git checkout fix/handle-unknown-alembic-revision
2. cp flexmeasures/data/utils.py /tmp/utils.py.bak && git show main:flexmeasures/data/utils.py > flexmeasures/data/utils.py
3. .venv/bin/python -m pytest flexmeasures/data/tests/test_utils.py -q
   Here the test fails, which is good.
4. cp /tmp/utils.py.bak flexmeasures/data/utils.py
   Here it passes.