Update magnolify to 0.9.8; adopt portable Timestamp encoding for IcebergIO

[shnapz]

• Bump magnolify from 0.9.7 to 0.9.8
• Switch IcebergIOIT from logical.millis._ (joda-backed DATETIME) to logical.timestamp.micros._ (Beam's portable Timestamp logical type at microsecond precision), which is what IcebergIO requires as of Beam 2.76.0
• Add LocalDateTime / unzoned timestamp coverage to IcebergIOIT (from Add unzoned timestamp type test to IcebergIOIT #5999)

Context

Beam 2.76.0 changed IcebergIO's timestamptz mapping from DATETIME to Timestamp.MICROS (apache/beam#39344). With the old millis._ import, reads now fail with ClassCastException: java.time.Instant cannot be cast to org.joda.time.Instant.

Magnolify 0.9.8 (spotify/magnolify#1392) adds logical.timestamp.{millis,micros,nanos} which map Instant to Beam's portable Timestamp at the declared precision. The old encodings remain available under logical.compat.*; the bare logical.millis/micros/nanos objects are deprecated aliases that preserve 0.9.7 behavior.

Verified locally

All three scenarios tested against a real Iceberg REST catalog container:

Import	Compat flag	Result
timestamp.micros._	none	pass
millis._	none	fail — ClassCastException on read
millis._	--updateCompatibilityVersion=2.75.0	pass

Test plan

• CI green (build + unit tests across 2.13/2.12)
• IcebergIOIT passes (already verified locally)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.99%. Comparing base (1d4bc82) to head (86949e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6000   +/-   ##
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  Coverage   61.98%   61.99%           
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  Files         323      323           
  Lines       11743    11743           
  Branches      839      839           
=======================================
+ Hits         7279     7280    +1     
+ Misses       4464     4463    -1     

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