Core: Include all reachable snapshots with v1 format and REF snapshot mode

[nastra]

I noticed that not all relevant snapshots are being retained with a V1 table and when using branches. I added a test that reproduces this issue and would fail with

Snapshot for reference SnapshotRef{snapshotId=7532742534704478452, type=BRANCH, minSnapshotsToKeep=null, maxSnapshotAgeMs=null, maxRefAgeMs=null} does not exist in the existing snapshots list
java.lang.IllegalArgumentException: Snapshot for reference SnapshotRef{snapshotId=7532742534704478452, type=BRANCH, minSnapshotsToKeep=null, maxSnapshotAgeMs=null, maxRefAgeMs=null} does not exist in the existing snapshots list
	at org.apache.iceberg.relocated.com.google.common.base.Preconditions.checkArgument(Preconditions.java:220)
	at org.apache.iceberg.TableMetadata.validateRefs(TableMetadata.java:835)
	at org.apache.iceberg.TableMetadata.ensureSnapshotsLoaded(TableMetadata.java:528)
	at org.apache.iceberg.TableMetadata.snapshots(TableMetadata.java:493)
	at org.apache.iceberg.BaseTable.snapshots(BaseTable.java:140)
	at org.apache.iceberg.rest.TestRESTCatalog.testTableSnapshotLoadingWithDivergedBranches(TestRESTCatalog.java:953)

This is because with V1 tables we removed non-relevant snapshots only based on timestamp.

[nastra]

this method only exists so that we have consistent error reporting in

iceberg/core/src/test/java/org/apache/iceberg/TestSnapshotLoading.java

Line 169 in 1521296

.hasMessage("Invalid table metadata: Cannot find current version");

for V1 and V2 tables (because we want to skip the validation of the starting snapshot id).

[rdblue]

I don't see a reason that we need to clear out newer snapshots for v1 tables. I think we were just trying to avoid the case where a newer sequence number causes a failure. It should be fine for a v1 table to have newer snapshots, since extra snapshots don't affect the current table state or branching. It's probably safer just to keep them all.