[BEAM-9999] Add Apex runner deprecation note.

[tysonjh]

Add Apex runner deprecation note missed in previous commit.

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Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

Post-Commit Tests Status (on master branch)

Lang | SDK | Dataflow | Flink | Samza | Spark
--- | --- | --- | --- | --- | --- | ---
Go | | --- | | --- |
Java | |
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| |


Python |


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| --- |
XLang | --- | --- | | --- | --- |

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[tysonjh]

R: @iemejia

[iemejia]

LGTM

[iemejia]

Thanks @tysonjh !