Change spot allocation strategy default from aws default to aws recommended value

[klingenm]

Today, the default spot allocation strategy is lowest-price, I suggest it be changed to price-capacity-optimized.

On AWS documentation on allocation strategies:

The rationale for having the default be lowest-price is that it's the default value on AWS.

I'm of the opinion, that a solution as wide and deep as terraform-aws-github-runner, should provide sane defaults where possible and no default where a decision is required by the user.

It can be quite easy to miss the fact that the default settings can cause periods of extreme flakynes.

Only reason I even found the allocation strategy was that I'm running a EKS cluster with spot instances and have way less interruptions on those instances, so I knew it should not be this bad. Now I know more, but if one does not have in-depth knowledge about spot fleets this setting is easily overlooked, as it is stated to be "aws default".

I have not been using spot instances too much before and was just about to switch over to ordinary on-demand instances when I found the setting.