As described in the release lifecycle docs from postgresql[1], v11 will
stop receiving fixes as of Nov 9 2023. This means it's EOL throughout
the entire lifetime of 23.11, so let's drop it now.
A lot of examples are also referencing postgresql_11. Where it's
sensible, use postgresql_15 as example now to avoid confusion.
This is also handy because the LLVM 16 fix for postgresql is not
available for postgresql 11 ;-)
[1] https://www.postgresql.org/support/versioning/
1220a4d4dd