RFR: 8150: SpotBugs entry in .gitignore hides flightrecorder.configuration files

[Alex Macdonald]

This PR addresses JMC-8150 [[0]](https://bugs.openjdk.org/browse/JMC-8150), in which the .gitignore entry to ignore the spotbugs generated configuration directories inadvertently ignores the files under `core/org.openjdk.jmc.flightrecorder.configuration/java/org/openjdk/jmc/flightrecorder/configuration`.

I noticed this when VSCode wasn't showing any of the flightrecorder.configuration files in my local file search, and it turns out that VSCode takes the .gitignore file into consideration when scanning for local files.

When running spotbugs on my Windows machine I see a bunch of generated directories which is what this .gitignore entry was aiming to ignore.

Example of generated directories:
![Screenshot from 2023-12-04 13-48-35](https://github.com/openjdk/jmc/assets/10425301/2dc857a1-cabe-4845-8f6b-518889a900c6)

This PR simply changes the current `**/configuration/` to `**/configuration/spotbugs` to make it more specific.

[0] https://bugs.openjdk.org/browse/JMC-8150

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Commit messages:
 - 8150: SpotBugs entry in .gitignore hides flightrecorder.configuration java files

Changes: https://git.openjdk.org/jmc/pull/535/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=535&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8150
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmc/pull/535.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/535/head:pull/535

PR: https://git.openjdk.org/jmc/pull/535