[charts/gateway] Independent pmtagger PDB values

[jmmAVGO]

Description of the change

Currently, the pmtagger PDB shares values with the gateway pods, like .create, .minAvailable, .maxUnavailable. Create a separate namespace by using .pmtagger.pdb.

Benefits

PDB for ssg and pmtagger can be separate, allowing ssg to operate with a PDB and pmtagger without.

Drawbacks

Change of existing named value

Applicable issues

• Addresses #US929445

Additional information

Have comments/questions for Gary - see below.

Checklist

• [X] Chart version bumped in Chart.yaml according to semver.
• [X] Variables are documented in the README.md
• [X] Title of the PR starts with chart name (e.g. [charts/gateway])
• [X] If the chart contains a values-production.yaml apart from values.yaml, ensure that you implement the changes in both files

[jmmAVGO]

@Gazza7205 this is the sequel to PR #259 , and a follow-on to our email on November 16th. I'm changing some of our internal scripts to only have one pmtagger pod, but as I do that, I need to separate the PDB so that if I fix one replica, a PDB sharing values with gateways/ssg won't prevent node shutdowns. This feels like it will work, but at the same time, is it the right approach? I'm also trying to be cognizant that external users may have lots of different needs and deployment scenarios!