Replace deprecated fetch_update with try_update once MSRV is 1.95 #7108

Rust 1.99 deprecates Atomic*::fetch_update in favor of try_update ("renamed to try_update for consistency"). With --deny warnings, this is a hard error in our clippy CI.

try_update was stabilized in Rust 1.95.0, but we support all rust versions less than a year old so we can't switch yet. As part of the 1.99 toolchain bump, the affected call sites will keep fetch_update and add #[allow(deprecated)] plus a TODO linking this issue:

• src/pubsub/src/publisher/token_bucket.rs: TokenBucket::try_acquire, TokenBucket::refill
• src/spanner/src/channel_pool/entry.rs: ChannelEntry::apply_error_penalty
• src/spanner/src/routing/server_connection.rs: ActiveRequestGuard::drop, ServerConnection::decrement_active_requests (test-only)

Once the MSRV is bumped to 1.95 or later:

1. Replace each fetch_update call above with try_update (same arguments and semantics).
2. Remove the matching #[allow(deprecated)] attributes and TODO comments.