Remove WAL-E and make WAL-G the default backup tool

[idanovinda]

This PR removes all remaining support for WAL-E from Spilo, finalizing the transition to WAL-G as the default backup and restore tool. WAL-E is no longer maintained and lacks support for modern PostgreSQL versions and storage backends, and doesn't support IPv6. In contrast, WAL-G is actively developed, faster, and more feature-rich, making it the preferred choice for Spilo going forward.

[zalando-compr-opensource]

invalid team ID

The team ID in your .zappr.yaml file (in the default branch) does not appear
to be valid. Please, fix this before team ID checks will be added back into
ComPR's specification check.

(If this PR is already fixing this, ignore the warning. But preferably fix it
in a PR separate from other changes, as the merge build from this PR will
not give a compliant image.)

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