Rename jobs count to builds in alive pipelines

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/356310

This MR is in preparation for the work in https://gitlab.com/gitlab-org/gitlab/-/issues/356310. In this MR we are renaming the method jobs_count_in_alive_pipelines to builds_count_in_alive_pipelines.

The reason is that jobs_count_in_alive_pipelines counts builds but does not include bridge jobs. For now we are not changing the behavior of Gitlab::Ci::Pipeline::Chain::Limit::JobActivity since we are deprecating this step in favour of a similar step earlier in the chain.

Database queries

• Existing jobs_count_in_alive_pipelines now renamed builds_count_in_alive_pipelines: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/12071/commands/42714

Time: 87.086 ms
  - planning: 2.210 ms
  - execution: 84.876 ms
    - I/O read: 84.326 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 14 (~112.00 KiB) from the buffer pool
  - reads: 9 (~72.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

• New method jobs_count_in_alive_pipelines (counting all jobs including bridge jobs): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/12071/commands/42715

Time: 1.805 ms
  - planning: 1.570 ms
  - execution: 0.235 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 23 (~184.00 KiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

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