docs: remove protocolVersion

[bpd0018]

The only supported backend/frontend protocol version is V3 since REL42.0.0 (committed Nov 14, 2016, released Feb 18, 2017). Removing it here makes this list consistent with pgjdbc/README.md in regards to protocolVersion.

[codecov-io]

Codecov Report

Merging #1128 into master will not change coverage. The diff coverage is n/a.

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  Branches       2531     2531           
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  Hits          10519    10519           
  Misses         3932     3932           
  Partials       1189     1189

[davecramer]

There are a number of other instances that should be removed as well https://github.com/pgjdbc/pgjdbc/blob/32c539020db3c940bae9b2a42425b1f23e864c73/build.properties#L21 https://github.com/pgjdbc/pgjdbc/blob/43e6505e3aa16e6acdf08f02f2dd1e3cf131ac3e/pgjdbc/src/main/java/org/postgresql/core/ConnectionFactory.java#L29 https://github.com/pgjdbc/pgjdbc/blob/5c12da16d3aa17d299e942c4a2b3647674422920/pgjdbc/src/test/java/org/postgresql/test/TestUtil.java#L53 https://github.com/pgjdbc/pgjdbc/blob/f6a727a96a437491813a8d940e1dbac287b53131/docs/documentation/faq.md#L153 https://github.com/pgjdbc/pgjdbc/blob/d7f0f271b73adbf0ae22146beea122e014d9f9f2/pgjdbc/src/main/java/org/postgresql/ds/common/BaseDataSource.java#L285 https://github.com/pgjdbc/pgjdbc/blob/e133510e70114db128784911b6790b5690d77320/pgjdbc/src/main/java/org/postgresql/core/Parser.java#L786 https://github.com/pgjdbc/pgjdbc/blob/32c539020db3c940bae9b2a42425b1f23e864c73/pgjdbc/src/main/java/org/postgresql/PGProperty.java#L54 https://github.com/pgjdbc/pgjdbc/blob/8be516d47ece60b7aeba5a9474b5cac1d538a04a/pgjdbc/src/test/java/org/postgresql/test/ssl/SslTest.java#L69

[bpd0018]

My thinking was to leave the actual code in place in case the fabled V4 protocol ever happens, but remove the documentation for the parameter so that people don't try to use it.

[bpd0018]

I have now updated faq.md as requested. Let me know if you agree about leaving the code as is.

[davecramer]

I've thought about this and if we are going to leave the code in then we should document the fact that it is currently irrelevant and why we are leaving it there.

[bpd0018]

I think there are two ways to do that - either just comment the property in PGProperty under the assumption that anyone looking at protocolVersion will find it there or add a comment at each of the locations where protocolVersion shows up. Which is preferable?

[vlsi]

@bpd0018 , there might easily be PGProperty.PROTOCOL_VERSION.set(properties, 3) usages in the wild, so we cannot just remove PROTOCOL_VERSION property.

[vlsi]

That is I would prefer to just document the current state of protocolVersion (== only 3 is supported, etc) and live it as is.

[davecramer]

@vlsi agreed