release-24.1: scplan: Fix deprules for dropping computed columns

[rimadeodhar]

Backport 1/1 commits from #120794.

/cc @cockroachdb/release

---

This PR fixes the new schema changer deprules for
dropping virtual computed columns which are also
used for hash and expression indexes.
Currently, the optimizer allows for virtual, computed
columns to be evaluated even when under mutation.
However, this causes concurrent DML issues when the
schemachanger job is running as the column that the
virtual computed column depends on moves into WRITE_ONLY
stage prior to the computed column being dropped.
As a result, the optimizer is unable to access the column
for evaluating the compute expression.
This PR updates the dep rules to ensure the virtual,
computed column is dropped before the dependent column
moves to WRITE_ONLY ensuring that the compute expression
can be enforced correctly for concurrent DML during all
stages of the schema change.

Epic: none
Fixes: #111608
Fixes: #111619
Release note: None
Release justification: Fixes a high priority bug that prevents users from writing to the table when a column with dependent, virtual computed columns is being dropped.

[blathers-crl]

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[cockroach-teamcity]

This change is 

[mgartner]

Reviewed 3 of 50 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @fqazi and @rafiss)

[fqazi]

Reviewed 50 of 50 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @rafiss)