chore: upgrade Gradle build system in generated library repo and fix setup issue

[chanseokoh]

Gradle 4.10.3 is ancient. We cannot delay upgrading Gradle forever.

Also, the following logic was incomplete in that it included directories (basically every non-hidden directory) that were not supposed to be included.

include rootDir.listFiles().findAll {
    it.isDirectory() && !(it =~ ".*/\\..*") && !(it =~ "^\\..*")
}.collect {
    it.getName()
}.<String> toArray(new String[0])

For example, a generated library always contains a directory called gradle/, and it was being included. (It's a minor point, but the regex ".*/\\..*" doesn't make sense to me either.)

Tested the changes by running bazel build google/cloud/compute/v1:google-cloud-compute-v1-java, unzipping google-cloud-compute-v1-java.tar.gz, and running ./gradlew check publishToMavenLocal.

[codecov]

Codecov Report

Merging #876 (bcf8d51) into main (a1fe20f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #876   +/-   ##
=======================================
  Coverage   87.84%   87.84%           
=======================================
  Files         153      153           
  Lines       15966    15966           
  Branches     1155     1155           
=======================================
  Hits        14025    14025           
  Misses       1601     1601           
  Partials      340      340           

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[vam-google]

@chanseokoh @meltsufin This change, since now the generated templates depend on gradle 7 broke the self-service workflow, because ./gradlew clean install does not work anymore (and there may be more issues).

[chanseokoh]

#884 will fix #876 (comment)