Firestore: Add @Deprecated annotation to getCollections

[schmidt-sebastian]

These methods are deprecated as per the JavaDoc but we should probably use an Annotation as well.

[BenWhitehead]

I'm okay with the additional detail, but the javadocs don't seem to provide an alternative of how the users is supposed to be able to do this sort of operation. Do you know what the replacement is that we could refer to?

[schmidt-sebastian]

The JavaDoc's deprecated comments do say to use listCollections(). GitHub decided that is not worth showing :/ Is there something else we should be doing?

[BenWhitehead]

Ah, so it does my mistake. LGTM.

[BenWhitehead]

Actually, can we change the deprecation javadocs to use this format:

 * @deprecated Use {@link #listCollections()}.

Instead of the back-ticks. This will make the link clickable in the javadocs if we do it.

[schmidt-sebastian]

Updated the JavaDoc to use the links as suggested.

[codecov]

Codecov Report

Merging #5916 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #5916 +/- ##
============================================
- Coverage 46.79% 46.79% -0.01% - Complexity 25648 25663 +15 
============================================
Files 2456 2456 Lines 267593 267593 Branches 30564 30559 -5 ============================================
- Hits 125229 125228 -1 - Misses 133105 133106 +1 
Partials 9259 9259

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/firestore/DocumentReference.java	73.25% <ø> (ø)	31 <0> (ø)	⬇️
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.54% <0%> (-0.35%)	40% <0%> (ø)

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