Deprecate Transliteration process plugin

Problem/Motivation

#3218356: Process plugin: service introduced a new generic process plugin - Service. This is a variation of Callback plugin provided by Drupal core, and can be used to execute a method of any available service, including Transliteration.

Now we can use Service process plugin to execute transliterate method of Transliteration process plugin. Benefits:
1) Ability to provide multiple method parameters using unpack_source feature;
2) Fewer files = easier maintenance.

Proposed resolution

Deprecate Transliteration source plugin and suggest using Service introduced in #3218356: Process plugin: service.

Remaining tasks

1) Agree on next release and release of removal (relevant issue: #3015034: [Plan] Create a new release);
2) Review / commit;
3) Add release notes snippets or change record;