Adds jenv's .java-version to project .gitignore (#317)

## What

* Makes git ignore [`jenv`](https://www.jenv.be/)'s .`.java-version`
file, to avoid adding it as an unintended artifact in future commits
* As of now, we do not explicitly encourage saving this file, nor do we
require [`jenv`](https://www.jenv.be/) to contribute to the project,
indicating that [`jenv`](https://www.jenv.be/) is the responsibility of
the user, instead of the maintainer(s)

## How

* Adds a rule to the project-level .gitignore file to ignore all
.java-version files

## Recommended reading order
1. Related to the initial steps in
[developing-locally](https://docs.airbyte.com/contributing-to-airbyte/resources/developing-locally)

## Can this PR be safely reverted and rolled back?
<!--
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* If that is not the case (e.g. a database migration), check NO.
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- [ x ] YES 💚
- [ ] NO ❌

.gitignore

@@ -16,6 +16,9 @@ data
 **/gmon.out
 static_checker_reports/
 
+# ignore jenv artifacts
+.java-version
+
 # Logs
 acceptance_tests_logs/