CI: remove fixed conda version
xref #18870 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=h1) Report
> Merging [#18994](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9c1071be085153d2b7d06417f5b2056ed86a8a70?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18994/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #18994   +/-   ##
=======================================
  Coverage   91.58%   91.58%           
=======================================
  Files         150      150           
  Lines       48972    48972           
=======================================
  Hits        44851    44851           
  Misses       4121     4121
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |


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[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=footer). Last update [9c1071b...6ed8a17](https://codecov.io/gh/pandas-dev/pandas/pull/18994?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger if this passes, then I think you can backport the skips closing in favor of #18995 