chore: Update layers.rst (#3389)

* fix(lambda-layer): drop unused libicu DT_NEEDED via --as-needed linker flag

AL2023's boost-devel puts libicu on the linker's library path. The default
ld behaviour records every library in scope as DT_NEEDED, so libparquet.so
ended up declaring libicudata/libicui18n/libicuuc as runtime deps even
though Arrow uses zero ICU symbols (confirmed by `objdump -T`). Lambda's
AL2023 runtime doesn't ship libicu, so layers for py3.12+ failed to
import with "libicudata.so.67: cannot open shared object file".

Pass -Wl,--as-needed via CMAKE_{SHARED,EXE}_LINKER_FLAGS so the linker
omits DT_NEEDED entries for libraries whose symbols are never referenced.
This is the proper fix for the original problem PR #3336 worked around —
it now applies generally rather than relying on libicu being absent from
the build env.

* [skip ci] chore: Update layers.rst

* [skip ci] chore: Update layers.rst

Author: kukushking