adapt to OpenSearch 2.x

[rursprung]

Description

DO NOT MERGE INTO main

this adapts the template to OpenSearch 2.x (currently: 2.15.0-SNAPSHOT).
as there's no 2.x branch on this repo yet i can only target the main branch. i'd suggest that you create a 2.x branch (based off of main; can only be done by maintainers) and then re-target this PR to the 2.x branch.

Issues Resolved

resolves #66

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[reta]

Ok, so one of the drawbacks of branches is that [Use this template] button does not allow to select a branch but will clone the repo (plugin template) with all branches [1]

[1] https://github.blog/changelog/2020-03-31-include-all-branches-when-creating-a-repository-from-a-template/

[rursprung]

Ok, so one of the drawbacks of branches is that [Use this template] button does not allow to select a branch but will clone the repo (plugin template) with all branches [1]

[1] https://github.blog/changelog/2020-03-31-include-all-branches-when-creating-a-repository-from-a-template/

i'm aware of this, however i think it's a good approach: when you create the repo from the template you have to toggle the checkbox that it should copy all branches and then you get main (targeting 3.xand2.x` (targeting 2.x) and can implement both of them - the plugin repo will after all follow the same branch pattern. and if someone only wants to target 3.x they can just skip the checkbox.

the only alternative i see is that you could instead create a complete separate repo for 2.x, but that'd not match what actual plugins do which support multiple versions with different branches in the same repo

[reta]

Thanks @rursprung , we need one more approval to get it in :)

[AmiStrn]

Thanks @rursprung !

[rursprung]

oh, i just saw this: this is a leftover from when i wrongly added the org.apache.hc.core5dependency and should've been removed again (since now we don't have any dependency here) :|

i can provide a PR for that next week (or someone else can briefly remove this). sorry about that!